Add section on removing message from holding pen.

Text adapted from https://github.com/mysociety/alaveteli/issues/465
Add a section on editing outgoing messages

Text adapted from https://github.com/mysociety/alaveteli/issues/465
Add a section on banning a user.

Text adapted from https://github.com/mysociety/alaveteli/issues/465
Add a section on deleting requests.
Use 'admin interface' instead of 'admin tool'
Add some basic instructions on changing the visibility of a request.
Add basic info on hiding incoming or outgoing messages.
Add some more details to the instructions for adding censor rules.