Add pagination to api/0.6/user/gpx_files

[mmd-osm]

As @systemed noted in systemed/potlatch3#34:

Ultimately then I guess this is an openstreetmap-website issue. As best I know, the /api/0.6/user/gpx_files call doesn't support pagination.

which seems to match: https://github.com/openstreetmap/openstreetmap-website/blob/master/app/controllers/api/users_controller.rb#L53

As some users might have a pretty large number of GPX traces, downloading all metadata at once might be quite time and resource consuming.

[tomhughes]

Please note that there is a ban in place on straightforward pagination and no PRs will be accepted which attempt to add it.

Pagination based on an indexed field like ID or a date that has been indexed is fine but a simple page number that refers to an ordered result set is not acceptable.