Add scheduled job to automatically update language criteria

[Rebreda]

I've tried to keep the PR small but I have refactored some older code to make it easier to use. Please read the comment in server/src/lib/jobs/sync-pontoon-statistics.ts to get an in-depth understanding of the logic being scheduled. In essence, Pontoon is a external dependency used to promote languages to be able to accept contributions. All existing logic is only run on startup, this PR aims to schedule a job that'll update language data periodically and decouple the progression of languages from startup.

I refactored how DB was instantiated and removed some other classes dependencies on DB - essentially turning it into a singleton, but the Node-y way (i.e. using a module). This means when called, the same instance of DB (now called "database") is used, making it easier to reason with.

I am also hoping to describe the schema used in the database via concrete objects and centralize how languages are accessed via the database (just like using an ORM). This needs more work though, so no guarantees I'll have it finished for this PR.