Feature/custom fields

[J4bbi]

This PR adds 115 custom fields using the mex: namespace.

These custom fields are derived from the 3.4.0 version of the MEx metadata model.

There is some validation implemented using little effort but this should not be required as incoming data will already have been validated using the model.

The mex:identifier field is the only required custom field.

The resource types vocabulary has been replaced with 11 MEx entity types. Please verify that these are the correct entity types.

The custom nested field class MultiLanguageTextCF is modelled on the MEx text field.

Templates

Three templates has been amended, records/detail.html which includes records/details/mex_details.html and records/macros/mex_detail.html only in order to list custom fields the same way Datacite fields are listed by default on the landing page.

The templates have also been changed to not expect a props property defined in the RDM_CUSTOM_FIELDS_UI config variable. In other words, if these amended templates were not used, Jinja would throw an error expecting each field in that config variable to have a props property set.

Unresolved

This PR does not have unit tests. They are coming soon in a PR for importing data.

There are still decisions to be made on the searchability of these custom fields, if and then how they are searched on or faceted.

[fdiehr]

The resource types vocabulary has been replaced with 11 MEx entity types. Please verify that these are the correct entity types.

I confirm, that these are the correct entity types.

[varadekd]

Verified the changes made, everything looks good to me.