Make Trac read-only

https://ghc.haskell.org should be immutable.

I will revoke the following permissions from the Trac user group to accomplish this:

• TICKET_APPEND
• TICKET_CREATE
• TICKET_EDIT_CC
• TICKET_EDIT_DESCRIPTION
• TICKET_MODIFY
• WIKI_CREATE
• WIKI_MODIFY

I have added a banner to point out that Trac is in read-only mode.

closed

added Trac label

I have also removed the following permissions from the trac developer group:

• TICKET_ADMIN
• WIKI_ADMIN