Allow sorting of fields with drag & drop

[jmartinm]

Currently, in order to move a field up/down, a user has to click several times on the up/down arrows which is not very convenient. Besides, this adds too many icons to the interface.

Solution

Implement drag & drop for reordering lists and add a small handle icon close to the fields that can be reordered.

Related to #169

[zzacharo]

zzacharo@edd5a83 initial implementation.

[harunurhan]

I have done an html5 implementation too but it lacks some UI feedback when you drag things around. So maybe it's better to use something like http://valor-software.com/ng2-bootstrap/#/sortable

[harunurhan]

It looks like we'll need to implement our own custom thing because neither sortable nor dragula works with Immutable.List out of the box.

[harunurhan]

@StellaCh @jmartinm we have to discuss about the final implementation. Do we want drag and drop for sure because there was this multipage limitation that we were concerned at some point?