[WIP] Use rails-bootstrap-forms gem for form rendering

This PR is related to #8470.

The goal is to use rails-bootstrap-form gem to simplify form rendering on bootstrap.

To do:

• Install rails-bootstrap-form gem
• Rewrite all form rendering with bootstrap-form helpers (admin section only for the moment, next with another pull requests)
• Wait for btn-group forms management: bootstrap-ruby/rails-bootstrap-forms#132
• Wait "HTML help block" issue answer: https://github.com/bootstrap-ruby/rails-bootstrap-forms/issues/211. Concern those forms:
  • https://github.com/gitlabhq/gitlabhq/blob/master/app/views/admin/applications/_form.html.haml#L17-L21
• Wait "Static control without label" issue answer: https://github.com/bootstrap-ruby/rails-bootstrap-forms/issues/212. Concern those forms:
  • https://github.com/gitlabhq/gitlabhq/blob/master/app/views/admin/broadcast_messages/index.html.haml#L14-L20
• Wait "Unwanted space with hide_label option on inline form" answer: https://github.com/bootstrap-ruby/rails-bootstrap-forms/issues/214. Concern those forms:
  • https://github.com/gitlabhq/gitlabhq/blob/master/app/views/admin/groups/show.html.haml#L61-L67
  • https://github.com/gitlabhq/gitlabhq/blob/master/app/views/admin/users/index.html.haml#L24-L28
• Wait "Unwanted hidden input on check_box helper with bootstrap_form_tag" answer: https://github.com/bootstrap-ruby/rails-bootstrap-forms/issues/215. Concern those forms:
  • https://github.com/gitlabhq/gitlabhq/blob/master/app/views/admin/projects/index.html.haml#L6-L24
• Wait "Ability to create radio button without label?" answer: https://github.com/bootstrap-ruby/rails-bootstrap-forms/issues/161. Concern those forms:
  • https://github.com/gitlabhq/gitlabhq/blob/master/app/views/admin/services/_form.html.haml
• Fix weird css issue: https://github.com/gitlabhq/gitlabhq/pull/9055#issuecomment-88881347
• Fix bad visibility form rendering on application settings: https://github.com/gitlabhq/gitlabhq/pull/9055#issuecomment-120387485
• Add some notes on UI best practices documentation [form_for / form_tag / alert_messages / f.primary]
• All tests should be green
• Update CHANGELOG file

Pending questions:

• Should we remove .form-fieldset class and add padding by default on fieldset element?
• Is the new form error rendering correct?
• Can Rubocop have custom rules detection? Could be useful to have a form_for usage detection, isn't it? (make bootstrap_form_for required. Use text_field_without_bootstrap is necessary)
• Should I use static control field for special subforms? (https://github.com/gitlabhq/gitlabhq/pull/9055#issuecomment-87835413)
  • No, use form_group instead.
• OK for using required label css trick? (https://github.com/gitlabhq/gitlabhq/pull/9055#issuecomment-89258863)

Externals links:

• https://github.com/bootstrap-ruby/rails-bootstrap-forms

Please tell me if I have some other task to do on this subject.

Thanks.