Fix wrong assignation of invited_by field on invitation creation process (fix #958)

Additionally it replaces the hack (django < 1.7) of double foreign key
on intermediate table (n2m relation) with trully foreign key (django 1.7
now supports it) adding corresponding migrations for conseve all previous
data.
Increment size of username and email field of user and add unique=True to email.
Add specific order fields for better handle of order in diffent sections.
Add migrations for model changes in previous commit.
Implemented the ability of mark some history entries as hidden.

Snapshots with order fields (that are considered not important
for preview) are automatically marked as hidden.
Fixed strange behavior on make_diff function (possible bugfix)
Add separated methods for bulk update order for: kanban, sprint and backlog.
Fix/Improve tests for new changes introduced in bulk order update process.
Add routing_key required parameter to base and postgresql events backend.
Improve events api for fulfill the new events protocol of taiga-events.
Send userstories change event on bulk order is changed.
Add rabbitmq events backend.