A user without 'view-c-room' permission is not allowed to see the
channel list. However, it was still possible to read and write
to a channel by guessing the name and modifying the URL accordingly.

Reading channel contents now requires either the 'view-c-room' permission
or being a member of the channel.

Due to this change, incoming webhooks user now need view-c-room
permission to send to public channels. We therefore add that permission
to the bot role.

Allows better matches on markdown formatted urls