To protect against potential abuse, the flag D is currently required
to run commands. As #wikidata-fr is dedicated for Wikidata work, and
there is no particular issue on this channel, this access control is
more a blocker for legitimate work than useful to stop vandalism.

This change removes this access control for whitelisted channels,
and keeps it in other channels.

Fixes T903.

[ New registry setting ]

• wikidata.channels: a list of channels where everyone is allowed to use Wikidata commands.

• areWikidataCommandsAllowed and isWikidataChannel tested with all the four scenarii (D user or not, in Wikidata channel or not).
• areWikidataCommandsAllowed tested not to query registry when the user has the D flag, to avoid a MySQL query.