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.