edit redirect to use current path

[JacksonXu33]

[Pivotal Tracker Link][tracker]

https://www.pivotaltracker.com/n/projects/2406982/stories/187044317

What this PR does:

This pull request fixes the resend_welcome_email redirect to use the original page of the user instead of a hardcoded path. The "Resend Welcome Email" button is in multiple locations of the app, so hardcoding a path would not work too well.

How should this PR be tested?

Already added RSpec testing, but manual testing can be done.

• What do the specs/features test?
  They test that the redirect_back works from multiple http links. They test for the two locations (that I know of) in the app that the button is present in and if redirect_back works.

• Are there edge cases to watch out for?
  Maybe if the user tries to run http://root/teachers/:id/edit/resend_welcome_email(email_format) directly into the searchbar, but even if someone does try that, the fallback will cover it.

Checklist:

• Has this been deployed to a staging environment or reviewed by a customer?
• Tag someone for code review (either a coach / team member)
• I have renamed the branch to match PivotTracker's suggested one (necessary for BlueJay) (e.g. michael/12345-add-new-feature Any branch name will do as long as the story ID is there. You can use git checkout -b [new-branch-name])

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.77%. Comparing base (d9d096d) to head (c64216f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #33   +/-   ##
=======================================
  Coverage   85.77%   85.77%           
=======================================
  Files          20       20           
  Lines         668      668           
=======================================
  Hits          573      573           
  Misses         95       95           

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[ArushC]

LGTM.