Refactor main process modules into multiple files

[micahflee]

I was starting Bluesky integration #342, but as an early first step, I decided to refactor various parts of the main process that were starting to get unwieldy, so I could more easily separate X code from Bluesky code.

So this PR includes the beginnings of Bluesky integration:

• New BlueskyAccount model and migrations, and update to the Account model to support a BlueskyAccount type
• Ability to select Bluesky account types in the main process (but not yet in the renderer process, so it doesn't show it as an option)

I think this should be fine to merge and release before we're ready for Bluesky integration.

But most of this PR is:

• Split shared_types.ts into multiple files in a shared_types folder
• Split database.ts into multiple files in a database folder
• Split account_x.ts into multiple types in an account_x folder

For database and account_x, the IPC functions (when the renderer process calls functions on the main process) should all work the same.

The tests all pass and it appears to work on my end, but I haven't thoroughly tested everything yet, so it's possible I've introduced new bugs with the refactoring. I'd love more thorough testing.

[redshiftzero]

I reviewed the diff commit by commit and the refactoring looks reasonable to me. Did some exploratory testing in a local build of the application and all seemed to work as before!