First bash at a redesign settings UX
Switch to tabs instead of subsubsub
Correctly highlight the tab for the first schedule
More work on redesign
made tabs responsive below 639px
commented out enable support button till we figure out what to do with it
Merge branch 're-design' of github.com:humanmade/backupwordpress into re-design
More work on redesign

- Adds a new inline upsell for the premium extensions
- Re-enables the support button
- Fixes up tabs
- Add schedule now works correctly
- Work started on Excludes overhaul, currently fatal errors
- Better error messages on settings form
- Some minor language improvements to the schedule sentence
- Schedule settings now work again
Switch to only storing 3 backups by default
Merge branch 'master' into re-design

Conflicts:
	admin/actions.php
	admin/schedule-form.php
	assets/hmbkp.js
	functions/interface.php
More work on Excludes

More of a proof of concept than a completed feature.

Uses wp-async-task to recursively calculate the filesize of a directory tree and cache the individual directory sizes.

The tree is not currently browsable and it you can't currently exclude items in the tree, nor are existing excluded items reflected.
Don't include the parent directory in a browsable list
Add some todos

Also stop ignoring the .git folder
Another todo
More work on excludes

- Styling for exclude list
- Correctly highlight unreadable and excluded files / folders
- Show filesize, permissions & type in directory browser
- Ability to browse into sub directories
- Ability to browse back up the tree
- Show the realpath on hover for symlinks
- Handle files / folders with a filesize of 0
Add a heading to the settings form
WordPress standard modal for enabling support
Show destinations in the list of schedule links

Rather than them being hidden under a mysterious "destinations" link.

Also removed the `$is_tab_visible` property and replaces it with a `has_form()` method on the parent class.

Introduces a `get_slug` method on the parent class which returns a sluggified version of the destination name.
Only show excludes for backups which include files
Remove uneeded $is_tab_visible