Improve supportability of JIRA/Conf backup managers

Currently the backup managers (both for JIRA and Confluence) allow backups to be taken as follows:

• once per 48 hours without attachments, and
• once per 48 hours with attachments.

Presumably this dual timer is to allow customers to generate a backup for migration testing, then still perform a full backup once they're ready for the final cut-over.

We are seeing the following pain-points with this current arrangement:

• From the customer's perspective, the time limit seems arbitrary and they are quite frustrated when they hit it.
• We don't say anywhere on the page that
  • backups can only be generated once per 48 hours (until they try it); or
  • backups with and without attachments are on different timers.
• The 48 hour timeout generally seems OK for the purpose of routine peace-of-mind backups, but not when migrating (which can be stressful enough as it is).
• Supporters have to run SQL and sometimes restart the application to clear the backup timer.

I propose that we make the following changes:

• Only limit the backup with attachments to once every 48 hours. The backup without attachments should not be restricted (new backups still overwrite old ones).
• Mention on the backup page that backups with attachments are limited to once every 48 hours, which they can see before they choose which type of backup to generate.
• Provide a button only available to system administrators that resets the backup timer, so that Support doesn't need to mess around with SQL.