Small performance improvements

[cjg89]

Description

• Added inline min-height, background-color to UCF Header placeholder elem to reduce layout shift on load.
• Added Customizer option for specifying additional domains that should be dns-prefetch'ed.
• Made some small adjustments in ucfwp_enqueue_frontend_assets() to ensure webfonts are enqueued as dependencies for the main theme stylesheet.
• Removed unused wp-a11y enqueue.
• Bumped jQuery to latest 3.x release.
• Added preloading of Cloud.Typography CSS key (if enabled) and Font Awesome font files (if enabled)
• Cleaned up some old update_option() calls to use option_${option_name} filter hooks instead.

Motivation and Context

• To implement some of the performance improvements recently added to the Main Site Theme
• Resolves #94
• Finally resolves #16

How Has This Been Tested?

Reviewed in Dev

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires an update to the documentation.
• [ ] I have updated the documentation accordingly.