Add GitLab UI form builder and custom checkbox field

📖 What does this MR do?

This MR adds a custom rails form builder for GitLab UI form fields (checkboxes, radio buttons, etc). This pattern allows us to reduce the amount of markup we write for each field and keep the markup consistent. Currently this MR only adds a custom field for checkboxes but this pattern could be extended to add fields for radio buttons and other GitLab UI components. To demonstrate how this custom rails form builder works this MR updates a setting in "Group" -> "Settings" -> "Permissions, LFS, 2FA"

🤔 How to use?

1. In HAML change form_for to gitlab_ui_form_for
2. Change f.check_box to f.gitlab_ui_checkbox_component and pass the label in as the second argument

= gitlab_ui_form_for @group do |f|
  .form-group
    = f.gitlab_ui_checkbox_component :example_field, _('Example field')

💻 Local testing

1. Create a group
2. Navigate to "Settings" -> "Permissions, LFS, 2FA"
3. The "Prevent members from sending invitations to groups outside of <group_name> and its subgroups." should look the same

📷 Screenshots

Expected to be the same

Before	After

🚦 Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
  • Not needed, developer facing change
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team