* Added 'writeapi' right that controls access to the write API. Users who don't have...

* Added 'writeapi' right that controls access to the write API. Users who don't have this right won't be able to use the write API modules even if $wgEnableWriteAPI = true; and they have the other rights required
* Checking for 'writeapi' in ApiMain::requestWriteMode() and tweaking the noapiwrite error message a bit
* Granting this right to *, user and bot by default.

For extra clarity: to e.g. move pages through the API, a user needs to have the 'move' right AND the 'writeapi' right AND $wgEnableWriteAPI = true;