Bug 17942 [Follow-up] Update style of the web installer with Bootstrap 3

Bug 17942 [Follow-up] Update style of the web installer with Bootstrap 3

This patch makes changes following QA comments:

- Login error messages now have a style
- Select all/Select none links should now be translatable
- Date format hint now correctly detects date format preference
- Unused JS is removed from onboarding.js

This patch also modifies the 'category_code_check' validation method to
allow numbers as well as letters.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>