Allow administrators to remove emails in prepare_error state

The current "Mail status" section of the wiki administration shows the various states of the emails waiting to be sent by XWiki.

When an email is being prepared for sending, errors in the preparation process can lead the email to be marked as the status prepare_error. For example, it's the case when XWiki fails to save the prepared email on the server filesystem due to a filesystem full.

In these cases, an administrator may want to not have this email being re-sent upon next wiki restart for various reasons. The only way to do so today is to manually execute a query on the database to remove the mail status entry that corresponds to the email that we want to discard, and to also cleanup the directory created for this email on the server filesystem.

One improvement to this process (which is technical and requires action from someone having access to the server) would be to provide buttons in the mail status section of the administration to discard some emails, especially the ones in an error state.