WIP: Update attach file and loading icon with GitLab SVGs and gl-loading-icon

What does this MR do?

Replaces the Attach a file Font Awesome icons with our own GitLab SVGs. That includes replacing the Font Awesome loading icon with the new GitLab loading icon.

Attach	Attaching

Important note - Instead of hide class, I replaced with hidden. That's because I used d-flex classes which use the conflicting display property. d-flex makes it easy to align icons, text, and buttons vertically, so that's why some parent divs contain d-flex align-items-center.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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