Add export (txt, csv, json) and import

[ldidry]

In txt and CSV format, the cards belongs to a column (if any).

The JSON export format is basically a dump of the database records for the board. This JSON export is suitable for import.

The import removes everything that is in the board and puts the JSON content instead.

Second commit adds a revision system. Push the button and a "snapshot" of the board is stored in the database, suitable for JSON export and so for import.

[ocdtrekkie]

Seems to work pretty well.

[ocdtrekkie]

The only thing maybe I would recommend is having these under like an expanding panel, rather than adding so much clutter to the board view page.

[ocdtrekkie]

I added a very simple jQuery accordion to your divs to make Import, Export, and Revisions into expandable sections rather than having so many buttons that are likely to be used rarely on the board view page.

ocdtrekkie@1b2fa0a

[Framartin]

This PR should include this fix to this issue I made, before being accepted.

[ldidry]

Fix added.

[alcalyn]

Thanks for this feature, I use it a lot on Framemo, and now I installed this fork on my instance because I need it.

I also developed a tool to import cards from Trello to a scrumblr board using this import feature : https://alcalyn.github.io/trello-to-scrumblr/

[Fabien3d]

Hello,
I'm new to Scrumblr and I tried the tool on scrulblr.ca.
Has the import/export function had been implemented? If so, how can I access to it?
Thanks!

[ldidry]

@Fabien3d As the PR is still no merged, and after a quick look at http://scrumblr.ca, no, the import/export is not present on it.