Split issue field forms into mixin

Each field form include one mixin, but it allows us to have a form with
many mixins, like the issue creation form
Make js for issue input fields independant from the form

To prepare use of each field method for the creation form which will
include all the fields
Issue numbers can be null (when created on the site)
Forms, views, urls, template to handle issue creation

Still need
- widgets
- job
- manage /created/ if issue created on github
- a button in addition to the "c" shortcut to open the creation modal
Activity ignores just created issues
Use select2 widget for the issue creation form
Display tags horizontaly in select2
Correct toggle-full-screen if many modals
Miss submit button in create form to validate with enter
Add job to create issues on github
Option menu for issues

A menu for easier access, with "toggle full screen" and "close" in the
menu
Also add "role=menu" on all dropdown-menu
And unload sticky waypoints (correct a bug with this menu, but also, and
especially, use less cpu by stopping checking for them, and free memory
by killing the waypoints, and possibly freeing the dom node ?)
Correct bug when returning to issue after creating/editing
Simplest creation form for simple users
Add a "add issue" button on the top bar
Add an option menu on create modal
Add refresh + view on github entries in issue menu
Add new shortcuts to the shortcuts window
Merge pull request #71 from twidi/create-issues

Create issues