Use wikibase-api post() not mw.api.postWithToken()

Take advantage of the wb.api.RepoApi.post() for token addition,
parameter validation and possible user checking. However, only
use it for changers since we need mw.Api for get() calls in other
widgets.

Bug: T209043
Depends-On: Ib69ab313239b62afc5df0dfa1b6149b4ae4d4b80
Change-Id: I3bede1e0d90a0f84c42f1c9863429a8ea66612e1