Add missing validations to the Hold model

[jazairi]

Why these changes are being introduced:

Thesis and Hold Source are required fields on the
Hold model. This is defined in the db schema, but
not at the model layer.

Because administrate checks model validations to
identify required fields, this means that these
fields don't appear as required in the admin
forms.

Relevant ticket(s):

• ETD-691

How this addresses that need:

This adds validations for Thesis and Hold Source
to the Hold model.

Side effects of this change:

None.

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

NO

[coveralls]

coverage: 98.252% (+0.002%) from 98.25% — etd-691 into main

[JPrevost]

Would you mind adding tests around a hold being invalid without a hold_source and without a thesis? I'm thinking mostly in case someone changes this in the future those tests should fail.

[JPrevost]

I think this got a bonus commit added?

[jazairi]

@JPrevost Sorry about that -- I was working with a bunch of different branches yesterday, and something must have gotten muddled. I just dropped the extra commit.