Implement changes from 7.x into 8.x

Done:

• Info text on admin/config/media/breakpoints added
• Weights are now saved for each breakpoint per group
• @media is now only editable on the 'global' settings page
• Cache is cleared after creating/deleting a group (replaced all usages of menu_rebuild() with menu_router_rebuild() according to this comment.
• Added permissions.

Todo:

• Add ctools export/import functionality.

This is currently not possible yet because the breakpoints are saved using the new config system. See #1649238: Implement CMI Exportable Controller.

Updated issue summary.

Updated issue summary.

Found 2 more while I was writing simpletests for d8, added them to the issue summary as well:

• make theme breakpoints overridable
• add interface for multipliers

Breakpoint is now in core...

Updated issue summary.