Handle paging in API calls

[saemy]

Both the message threads as well as the messages per thread are paged in API calls. The app does not handle this.

And the REST replies are weird. Messages of a thread are returned more in a paging mode rather than in the expected limit/offset way:

1. Have a thread with 51 messages
   2a) /services/rest/message/getThread/?thread_id=xxx gives you the 50 most recent messages
   2b) /services/rest/message/getThread/?thread_id=xxx?offset=0 gives you messageCount-50 messages starting at the oldest message (i.e. with 51 messages giving us exactly the oldest message)
   2c) /services/rest/message/getThread/?thread_id=xxx?offset=1 gives you 50 messages starting at the second oldest message

The message threads do not seem to support the offset parameter at all (while respecting the limit).