[Release]: Theme cleanup + improvements

[rediris]

What this PR does:

• removes unused SASS files
• implements recommended style revisions in JS files
• shuffles all meta to a new gym-head-meta block
• updates color scheme in password reset email
• removes favicon (in favor of serving it up from gymcms)
• comments out some JS (loading from gymcms)
• updates tab logic
• resolves footer CSS errors on login screen (due to a non-standard inclusion of the footer): see https://github.com/gymnasium/tracker/issues/279
• resolves issues with missing password reset page CSS (https://github.com/gymnasium/tracker/issues/289)

NOTE: this PR must be deployed to production along with the gymcms release: https://github.com/gymnasium/gymcms/pull/810