Allow users to assign bookmark folders as the bookmarks toolbar per-space

[jakelauer]

I think, rather than playing whack-a-mole with the bookmarks bar workspace bugs (a la #9080), a more elegant solution would be to allow users to assign a bookmarks folder to each space for its toolbar, and then switch out the bookmarks toolbar contents when switching spaces. Doing so would have a ton of benefits:

• Eliminate all inter-space ordering complexity
• Enable space-specific separators and separator positions
• Allow space-specific labels for the bookmarks of the same website (currently this would require you to make multiple identical bookmarks with different labels and then change which spaces each one appears in)
• Allow users to see and access the other toolbars' contents if so desired, by opening the Other Bookmarks folder
• Enable normal use of the Bookmarks Manager without requiring any design or functionality changes
• Retain all of the built-in bookmark syncing, including to Firefox mobile
• Retain compatibility with Firefox itself, as the bookmarks toolbar folder is just a folder. Firefox would have all of the custom Zen toolbar bookmarks folders in its Other Bookmarks and would show the bookmarks toolbar as normal.

I believe this would eliminate significant developer toil and bug management for this feature, and given the current buggy state, I think it'd be much better to pivot from the design early before it becomes much more complex.