Add option to name backups

:+1:

Discourse backups are super easy to make and restore, but their names (timestamps) can be cryptic when you make backups often while changing various things on the site. I can’t remember precisely which of these is the backup before upgrading to 1.5.0 beta, and which is the one before I ran an API script.

quantified-self-forum-2015-09-23-083352.tar.gz	8.9 MB	Download     Restore
quantified-self-forum-2015-09-23-040000.tar.gz	8.7 MB	Download     Restore
quantified-self-forum-2015-09-22-094309.tar.gz	8.7 MB	Download     Restore

Labels/notes for backups would be great.