Start- and Duedate for cards

[shoetten]

As discussed in #12, here is my take on start and due date functionality for Wekan.

I updated the original pull request a little bit, but these points are still valid:

• Hook into the wekan activities / notifications feed. A reminder message e.g. 2 days before an assigned card expires would be awesome..
• Make the moment locale change reactively, so the date format changes when the user selects a new language.

Also, i would suggest putting the badges before the members in the minicards, but since that's more of an aesthetic opinion, i did not include it in this pull request.

Some more thinks to consider:

• I now used the atmosphere version of bootstrap-datepicker. I would have preferred to use the npm package, because atmosphere only has an outdated version available. But since Wekan is not really set up in a npm / ES2015 kind of way, the atmosphere package was the easiest solution and it gets the job done.
• mquandalle:moment is stuck at v2.9.0 of the moment.js library, which does not support calendar time formating. Fortunately it degrades gracefully, so it will just start working, once a newer moment.js is used. It might be a good idea to use a different moment wrapper, like this one. That would require a small rewrite of how dates are currently displayed, though.

Additionally, as @xet7 pointed out, it would be nice to be able to import due dates from Trello.

If you're ready to merge, i can squash it done into a single commit, if you prefer it that way.

[xet7]

@shoetten Thanks!

[jLouzado]

@shoetten I'm working on a similar feature and used this PR as a reference because I'm new to the wekan codebase and needed to start developing relatively quickly.

I'm trying to implement another field (together with 'edit' popups) to show an 'Estimate # of Hours) for a card. I'm stuck at the point where my 'edit' templates are not displaying; I'm having trouble with the Popup.open() system in Wekan. My code is here, I'd be very grateful if you could take a look at it.

If you'd like to get in touch with me I'm on the wekan channel chat as well: joel.louzado.