css refactor: components/_social

[Bargraph6]

What are the relevant GitHub issues?

related to this PR: #5212

What does this pull request do?

This PR:

1. renames _socials.scss to _socials.css
2. updates the social icons to use css variables instead of scss variables
3. adds a .visually-hidden css class to use in place of the current text-indent: -10000%; approach

How should this be manually tested?

I loaded up the home page and articles page to see the social links:

	before	after
home
article

Questions for the pull request author (delete any that don't apply):

• Are any needed README/wiki/documentation updates needed for this pull request?
• Does the code you updated have tests? If not, could you add some please?
• Does this pull request require any new server dependencies which need to be added to the build process? If so, please explain what.

Acceptance Criteria

These should be checked by the reviewers

• This pull request does not cause the database export script to become out of sync with the db schema

[Bargraph6]

@veganstraightedge @just1602 this is ready for review

[Bargraph6]

In the github UI, if you look at the individual commit diffs, rather than the "all commits" view, you will get a more helpful diff:

f60514c