Shared with me

This re-works our file tree and workspace to uplift pending-shares to first class citizens in lockbook. Presently if I want to start collaborating with someone they have to:

1. find the lil icon that relates to pending shares
2. understand what "accepting a share" means
3. find a place to put that share within their lockbook

before they can even see what they're accepting.

Now all their shares will just hang out in there, indexed by who shared it, sorted by what what changed the most recently. There is no longer any obligation to accept a share and they can read / write any of the things they have access to.

I also changed the suggested docs icon to ✨ (kinda) and this is what it looks like when shares are collapsed:

#3511 on linux & windows.

Remaining work

in the order that I plan to do it.

• other platforms is high on my list, have been in talks with @ad-tra and his design guidance has been great. Will be talking soon with @smailbarkouch this weekend and potentially we can pair on getting apple done as soon as possible.
• for now the keyboard just skips over this new section. There's some trivial algo stuff to do resolving the idea of parents & siblings that would make full keyboard navigation possible.
• This PR also does a fair amount of file tree cleanup along the way to make room for the additional complexity. Now that I understand the drag & drop system, I can make just the top level shares draggable as a really nice way to accept shares.