Paginate the get sentences API

[allan-simon]

use a "last_id" type of pagination

[jean553]

Should we do the same for GET /languages/{language_id}/sentences ?

[allan-simon]

yes , should be tracked by a separate issue I think

[jean553]

Is last_id optional ? In fact, Rocket does not handle optional query parameters. A PR has been provided a few weeks ago here rwf2/Rocket#454. Should we add a second route or simply require the last_id as a mandatory parameter, with a default value for the first page ?

[allan-simon]

by the way in an other API I've seen people using "starting_after_id=" for pagination, which i think was more "readable" than "last_id"

I should be optional