Add tracks pagination

* add "limit" and "offset query params to /tracks.
* use doctrine paginator to properly paginate fetch joins.
* Don't make useless requests when all tracks are displayed.
Left-join Track's tags.

Otherwise tracks with no tags aren't in the results. This should never
occur though, as tags are a required non-null field. But working with
legacy v0 data right now...
Merge pull request #16 from kimlai/pagination

Pagination