Increase performance of the readinglist

[Draculente]

Although it is really hard to test, it appears like the really big readinglist of a single users was the cause for a quite stark drop in performance (over the whole instance) whenever the user opened the readinglist.
This drop in performance was, in fact, so big, that the health check for the instance failed.

I have some ideas to mitigate those issues in the future:

• Paginate the readinglist
• Set a default "time-to-disappear" for unread and read articles in the readinglist
• Set a limit of articles one can add to the list