Clear previous backup notices when running a new backup
Minor refactor for PHP minimum requirements notice

- Less generic function names
- No need to split link into the 4 sprintf parts
Refactot HMBKP_Notices to support non-persistant notices

- Add support for non persistant notices and uses that for the notices defined in interface-functions with the `server_config` context, fixes #646
- Removes the dismiss button for those notices, fixes #647
- Escape server_config notices with `wp_kses_data`
- Wrap generic backup errors in `<pre />`
- remove hardcoded reference to `wp-content` in one of the error messages, fixes #641
Merge pull request #649 from humanmade/improved_notices

Improved notices