Refactored 'users' module

- allows editing users (currently the groups and username)
- allows any logged-in user to change the passwords of any other users
- improved url highlighting of subsubmenu
Fix subsubmenu highlighting bug and add tests
os.path.commonprefix is suitable for any strings
Remove confusing sidebar message during user creation
Update UI strings in user module

- Fixed an i18n issue.
- Marked more messages for i18n.
Update user module URLS

- It is more restish to have /user/1/delete rather than /user/delete/1.
- End all URLs with a slash.
Auto-generate model form for user updation

- Remove the custom UserForm
Show active status in user list, increase delete button size

- Separate out styling details
Don't mark ContextMixin as abstract

- Django abstract bases seems to apply only for Models.

- Django itself does not use abtract marking for its own generic view
  mixins.
Sylting updates in user module for consistency
Refactor templatetags and tests for styling