Small fixes, specially in the frontpage forms

- Inputs on the forms on the frontpage a little wider
- Select2 more exactly the size of other inputs
- Fix labels in the form in registrations/new
A lot of small improvements, mostly visual

Includes:
* Better style for .thread, cleaner
* Better list of users for admission requests and join requests
* Only show navigation div when there is actually buttons to show
* A few pages in spaces were not including the sidebar stylesheet
Prevent public activity views from breaking for disabled users

If a user that generated an activity is disabled, now it shows only the user's
name, not a link to his non-accessible profile.
Generate disabled users and spaces in db:populate
Adjustments for posts and priv messages by disabled users

Prevent breaks and show information that the user was removed.
Fix permissions for space admins over join requests
Fix failing test introduced by be47d4f83a66