Allow setting a ?limit parameter for /api/user/feed

This is related to #700.

It allows setting a `limit` GET parameter for `/api/user/feed` so you can get more than just the last 5 recently changed repositories.

This definitely needs some code review as it is my first "bigger change" with Go and it feels incredibly backwards and inefficient (getting the parameter, parsing it to `int64`, then converting it to `int`).

Coming from proper Web frameworks like Django or Flask this feels very inefficient, error prone and not reusable at all. It would really suck to add more parameters to other routes like this.