Workspace admins should be able to set the name of the admins channel

[billglover]

Not all workspaces will have an admins channel called "admins". BuddyBot should allow administrators to customise the admins channel for a workspace.

This functionality must:

• notify the admins when the channel is changed
• prevent non-admins from changing the admins channel
• persist the admins channel name once it has been changed