Feature/fields refactor

The Field Refactor branch is getting ready to be merged into the devel branch.

I am going to open this pull request to keep track of it's work in progress. I expect that by the end of today this will be ready to be reviewed.

Relevant tickets are:
https://github.com/globaleaks/GlobaLeaks/issues/1006
https://github.com/globaleaks/GlobaLeaks/issues/912

I am not sure how much other stuff in here:
https://github.com/globaleaks/GlobaLeaks/labels/OTF-D4.2:%20Introduce%20configuration%20profiles%20(ready-made%20submission%20fields)
should be fixed by this branch.

@evilaliv3 @mmaker