Support auto sync with dropbox

[jackycute]

Some people use dropbox in their workflow.
Would be good if we can auto sync with a specific file and make revision of it in dropbox.

[eshao]

+1, this would also help for backups!

[ccoenen]

seems also related to #275

[RoyiAvital]

Yep, Connected App mode to Dropbox will be great!
Hopefully with support to Restricted Mode of Dropbox (Namely accessing only Application Folder).

I think some of it can be borrowed from @stackedit.
Though it seems StackEdit's implementation isn't robust.

[brlin-tw]

This is rather helpful after the 228 storage fault incident. If possible I'd really like it to sync it with external Git repositories (and, if possible, retain the edit history with every author's identity).

[brlin-tw]

Also, if possible again, sync with the local installation of HackMD, like hackmd/docker-hackmd so that user can always has a local backup.

[ccoenen]

@Lin-Buo-Ren in that case #218 (Sync to Git(hub)) is the right ticket for you :-)
I guess syncing to another HackMD didn't come up yet, but it would probably also be best to make this its own ticket.

[brlin-tw]

@ccoenen Thanks for the pointers, #114 seems to be suitable as well.