fix(theses): use Whooshee for full text theses search

[true-real-michael]

Use Flask-Whooshee instead of flask-msearch for full text search

Issue #92 mentions the broken search engine at https://se.math.spbu.ru/theses.html
Issue #74 mentions the deprecated technologies used for full text search.
This PR replaces flask-msearch with Flask-Whooshee, which is more actively maintained and works in this case.

Changes made:

1. Removed flask-msearch dependency withFlask-Whooshee.
2. Replaced__searchable__ = [...] attribute with @whooshee.register_model(...) for Users and Thesis models.
3. Replaced msearch with whooshee initialization.
4. Replaced msearch(search) with whooshee_search(search) in fetch_theses().
5. Added indexing when starting the app.

[iakov]

Thank you very much!

[true-real-michael]

Reopening this because @iakov has approved the changes.
However, I do not have the resources to extensively test the solution and the potential edge cases.