Implements file-upload via dropzone

[jacargentina]

This fixes #2071.

It adds dropzone js library; and also updates boostrap-markdown to v2.9.0
Also includes font-awesome for the bootstrap-markdown icons.

[mxstbr]

We've already got Octicons in Keystone, why do we need another icon pack?

[jacargentina]

I was trying to use upstream bootstrap-mardown. That implies not doing nasty hacks.

From bootstrap-markdown docs:

The icon library to use. Glyphicons (glyph) and Font Awesome (fa) are supported. In order to use Font Awesome properly, you'll need to include Font Awesome stylesheet yourself. Default to glyph

Maybe you would like to PR on them with adding octicons as an option?

[mxstbr]

I don't think sending down another icon set just to get drag-n-drop upload working is worth it. As much as we appreciate you taking the time to submit this PR, if there's a solution without font awesome that'd be a lot better!

[jacargentina]

It serves my need. The branch will be there as reference on my fork if someody really needs it (just works)

Cheers!

[mxstbr]

Awesome, that's great to hear! We really appreciate you taking the time to implement this, sorry if I came off a bit harsh. 👍

[jacargentina]

@mxstbr
Please look at this elementalui/elemental#146 which depends also on refactory-id/bootstrap-markdown#232

New PR is keystonejs/keystone#2508