Replace remaining fontawesome icons with emojis

What does this MR do?

This MR replaces the remaining fa- icons with emojis. Those icons will only be used on the following pages:

• Wiki
• Project repository
• Project snippets

All these pages use the ascii_doc_sanitization_filter.rb which requires Fontawesome, see #267127 (comment 431007269) for details.

In addition, this MR removes both the Fontawesome font and the gem from our codebase.

Screenshots (strongly suggested)

fa icon	header
fa-exclamation-triangle
fa-exclamation-circle
fa-lightbulb-o
fa-thumb-tack
fa-fire
fa-square-o
fa-check-square-o

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

Relates to #267127 (closed)