Scope the fontawesome.css to the icons we actually use

What does this MR do?

We only use a limited amount of Fontawesome icons anymore but we still automatically include the CSS for all icons.

This MR adds fontawesome 4.7.0 as a frontend dependency and introduces a custom CSS version that only includes fa- classes which are currently used in our codebase.

Version	Size
Full CSS from gem	38kb
Custom CSS	5kb

Screenshots

N/A

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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

Closes #229466 (closed)