Use GitLab SVGs for admonition icons

An MR that builds off functionality added in: #649 (closed).

Allows icons from: https://gitlab-org.gitlab.io/gitlab-svgs/ to be used for admonition boxes instead of Font Awesome.

In many ways, this supports an epic where this is being done in the GitLab product itself: &2331 (closed).

• Before: https://docs.gitlab.com/ee/development/documentation/styleguide/#note
• After: http://eread-use-gitlab-svgs-for-admonition.178.62.207.141.nip.io/ee/development/documentation/styleguide/#note

Preview of changes

• Note:

  

• Tip:

  

• Caution:

  

• Danger: