Fix bugs around thesis graduation date

[matt-bernhardt]

** Why are these changes being introduced:

• There is a bug in the thesis model which prevents updating thesis
  records via the admin dashboard. We specify graduation dates in the UI
  via separate month and year fields, but the information gets stored
  internally as a unified date. There is a method to build this date,
  but it only called on record creation - not when the record is updated
• Additionally, the admin dashboard currently imposes conflicting
  requirements on the graduation_month field, being both a number and
  the name of the month.

** Relevant ticket(s):

• https://mitlibraries.atlassian.net/browse/ETD-216

** How does this address that need:

• This updates the thesis model to call combine_gradaution_date every
  time the record gets saved, not just for initial creation.
• This change also adds unit tests to confirm that this happens on both
  creation and save.
• Another change is to make the admin dashboard treat months as a text
  string, rather than a number. Ultimately it might need to be a
  dropdown, but we can start here.

** Document any side effects to 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 remained the same at 94.476% when pulling bbc58c3 on etd-216-grad-date-bug into a0a1e72 on main.