add some search functionality

super early WIP, resolves #13

this requires Meilisearch to be installed. it's ridiculously fast and super easy to use. eats up a lot of disk space however but i guess that's what it takes to have a decent search.

to set it up locally (macOS):

brew install meilisearch

to enable search functionality:

SEARCH_ENABLED=true foreman start -f Procfile.dev

to import data to meilisearch:

SEARCH_ENABLED=true bundle exec rails runner '[Answer, Question].map(&:reindex!)'

what's still left to be done:

• [ ] decide how we want to present the search results (maybe each type in a tab? multisearch, while fast, seems a bit wonky)
• [ ] decide whether we want search to be done by signed in users only (i'd say yes)
• [ ] decide what questions to include in the index (answer count >= 1? non-anonymous ones? definitely skip the generated ones though)
• [ ] pagination
• [ ] clean up the UI in general (this includes maybe a search bar or button in the navbar)
• [ ] set it up to index stuff in a sidekiq worker (see also https://github.com/meilisearch/meilisearch-rails?tab=readme-ov-file#queues--background-jobs)
• [ ] perhaps introduce some /@user/questions/search / /@user/search pages too which only searches for the user's qustions/answers
• [ ] add some gradual rollout so we can keep an eye on how it behaves
• [ ] deal with extra user flags (exclude from search results)
• [ ] introduce a flag for enabling/disabling search globally
• [ ] introduce a flag that overrides the user's choice of being present in the search results (the idea is to not display results from spammers or some other problematic people)
• [ ] (for prod): set up machines for meilisearch