Restrict user to use accented characters in username

[asadiqbal08]

Pre-Flight checklist

• Testing
  • Code is tested
  • Changes have been manually tested

What are the relevant tickets?

fixes: #259

What's this PR do?

(Required)

How should this be manually tested?

Create a user with username that may have accented characters in it e.g. önlínê-user. The system should not allow it.

NOTE:

- I did not update the message Username can only contain letters, numbers, spaces, and the following characters: @_+- , seems of enough but if needs to modify then let me know.

[codecov-commenter]

Codecov Report

Merging #262 (cc5e5cb) into main (878a9a0) will decrease coverage by 4.72%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #262      +/-   ##
==========================================
- Coverage   91.00%   86.27%   -4.73%     
==========================================
  Files         214       93     -121     
  Lines        6338     3542    -2796     
  Branches      314      314              
==========================================
- Hits         5768     3056    -2712     
+ Misses        483      399      -84     
  Partials       87       87              

Impacted Files	Coverage Δ
users/serializers.py	94.83% <100.00%> (ø)
...ic/js/components/forms/RegisterDetailsForm_test.js
static/js/lib/validation.js
...ntainers/pages/register/RegisterDeniedPage_test.js
static/js/components/forms/RegisterEmailForm.js
static/js/contextDefinitions.js
static/js/constants.js
static/js/components/TopAppBar_test.js
static/js/lib/validation_test.js
static/js/factories/util.js
... and 112 more

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[arslanashraf7]

@asadiqbal08 You might need to re-run the tests since the CI is failing.

[arslanashraf7]

Although functionality wise it works fine and doesn't allow accented characters, I have two minor things that you might consider here:

1. I'm sure there could be a reference to using the space in the username which is unusual but if we do want to use it we should place that somewhere close to the allowed special characters list e.g. r"[A-Za-z0-9 .@_+-]+"
2. Since we are matching the username text through a regex, shouldn't we add this regex validation on the frontend validator(https://github.com/mitodl/mitxonline/blob/main/static/js/lib/validation.js#L20) and we can save us an API call that is just going to tell the users about what characters they can use after hitting the submit button.

[arslanashraf7]

I know point#2 might not be purely related to the PR but that has a chance to get fixed in this, @gsidebo could even give a better thought on this.

[asadiqbal08]

@arslanashraf7 regarding the point.2,

• It makes sense to me that there should be frontend validation earlier before sending an api call.
• space in username does not make sense to me as well. I keep it by considering it may be a requirement earlier but I have removed it and take input from @gsidebo if space really makes sense to be part of username.

kindly take a look over another commit.

[gsidebo]

We currently have usernames with spaces. I don't think we need to remove that capability

[gsidebo]

Just FYI I'm labeling this "on hold" for a few reasons:

1. This doesn't help us for usernames that already have accents. If someone has already signed up as üser, that is their username in mitxonline and in Open edX. If another user comes along with the username user, that will trigger this same bug behavior. For this PR to avoid that issue as it stands now, we'd have to mass-update usernames to get rid of the accents. I want to see if we can avoid that.
2. I don't know if there's a great UX for telling users that they can't use accented characters. The message as it is now is not clear enough (Username can only contain letters, numbers and the following characters: @_+-).
3. If we can't figure out the true source of this bug, I think it probably makes more sense for us to add a new field to the mitxonline User model which will store the un-accented version of the username. That way a user can use accents in their username without an issue, but we can prevent 2 users from using the same username with different accents by checking against that new field

[gsidebo]

Closing this so we can re-implement according to the details in the updated issue (#259)