Add missing paper report button

[hexhowells]

Some papers don't get added to the database when automatically fetching from arxiv.org for some reason. A possible solution (other than figuring out what's causing these missing papers) is to include a "Report missing" button/page for any paper page that returns blank.

A user can report the paper as missing in the database, and this can instantly and automatically be checked and added to the DB by the server, in order to be added the request should fulfil the following conditions:

1. The paper doesn't exist in the DB (the issue could be something else)
2. Check that the paper exists in arxiv
3. Check that the paper falls within one of the currently supported categories

If the above conditions are met then the paper can automatically get fetched, processed, and added to the DB. The user should see some loading screen indicating that the server is performing this and redirect the user if the paper is successfully added.

Rate limiting and abuse potential should also be considered here, since a single request can cause the server to do a fair bit in the background.