Adds the initial backend requirements for the
OAuth flow, which will be made required for all
search engine checks. This adds three new routes:

- `/login` (GET, POST) - for logging in
- `/logout` (GET, POST) - for logging out
- `/oauth-callback` - OAuth 1.0a callback route

Login/logout state can be checked through the
header. By default, a separate page navigation is
not required when the link is clicked by the user.
When following a link to `/log(in|out)`, however,
an extra button will be shown to prevent inadvertent
logins/logouts.

- All POTD logic is now in Python instead of JS.
- Removed EarwigBackgroundCache cookie (now stored in a new `background` table in the database).
- Added EarwigScreenCache cookie
- Increase cookie life from a year to three.

- Also removing some unused imports and cleaning up code.