feat: User Profile API endpoint

[fkulla]

What

I propose adding a new API endpoint GET /api/user/profile that responds with information about the authorized user's preferences/capabilities in the following format:

{
  "theme": "auto",
  "bookmark_date_display": "relative",
  "bookmark_link_target": "_blank",
  "web_archive_integration": "Disabled",
  "enable_sharing": false,
  "enable_favicons": false
}

This mirrors the internal model of the UserProfile, but I think it would be better to return the web_archive_integration state as a boolean for convenience. Maybe it could be renamed to enable_web_archive_integration?

Why

To allow API clients to query user capabilities. It's a prerequisite for changes needed at sissbruecker/linkding-extension#27.

[fkulla]

Would like to self-assign this issue. Not familiar with Django, but it seems like a simple addition.