Export the db structure (of a group) so I can setup my own crabcrass

[gerdneuman]

Spawned from #236 (comment):

Main idea is to be able to export one's (group) dataset in order to import it into another crabgrass instance.

The ability to export one's dataset makes using the hosted Crabgrass instance much more approachable because it means that one's data is not forever in some format that is only meaningfully accessible if the remote service is available.

Further, I work in some environments where the absense of a local backup precludes the use of Crabgrass by policy. Adding this feature would help me make the case to certain groups to move to Crabgrass away from less safe and less philosophically aligned alternatives such as Gogle Docs.

See also the many, many dupes of this feature request:

https://labs.riseup.net/code/issues/475
https://labs.riseup.net/code/issues/754
https://labs.riseup.net/code/issues/1270
https://labs.riseup.net/code/issues/1702
https://labs.riseup.net/code/issues/4099
https://labs.riseup.net/code/issues/4419

TL;DR: Please implememt this feature regardless of commitments to keep we.riseup.net alive. It is an important data portability issue and software libre issue on its own merit. Thank you!

@meitar cc