Add LinkedToUserForm and LinkedToUserFormView on front/

These two classes are useful when the form needs a user to validate/save
an object (ie attributing the creator...)

- LinkedToUserForm save the "user" entry given in kwargs in its "user"
attribute
- LinkedToUserFormView is for FormViews for with forms based on
LinkedToUserForm, which will automatically pass "request.user" to the
form.
Rework issue-edit-state form, view, template

Prepare for the main issue edit stuff
Rewrote views and front mixins and put them in front.mixins
Allow specifying a timeout per api call
Rework issue-edit stuff to work field by field

and
- allow passing values to use in dist_edit
- manage errors when editing state
Issue's title is editable !
Block editing a field while a job for this field is waiting/running
Stop fetching issue after editing, but queue a fetch job
Issue's body is editable !
Discad the "big-infos" block if empty
More room for the edit-body button
Restore repo switcher
Colorize issue number depending of its state
Issue's milestone is editable !
Max width for involved, new icon for assignee
Move the assignee under the milestone
Issue's assignee is editable !
Correct canceling issue editing in modal
Issue's labels are editable !
Merge pull request #65 from twidi/edit-issues

Edit issues