allow saving to a server

[DanielFGray]

should improve sharing by link instead of only being url-encoded.

ideas:

• generate hash sums of playground snapshots and use for storing commit ids

• prune old/free/anonymous playgrounds
  retention = min_age + (-max_age + min_age) * pow((file_size / max_size - 1), 3) ^source

• allow namespacing and vanity urls

• needs client side router #23

• use scm api to allow tracking revisions of notebooks #24

[DanielFGray]

backend started in 51a50fd
does not yet have UI or endpoints for actually saving

need to make sure that loading from url doesnt override "default workspace" where user may have unsaved changes?