Update/mobile nav menu fade

[Spleeding1]

Description

Addresses issue #805

Updated navigation CSS to prevent blocking of content when mobile menu is toggled off and to have smooth transition from toggle-on toggled-off states.

List of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] This pull request relates to a ticket.
• [x] This code is tested.
• [x] This change has been added to CHANGELOG.md
• [x] I want my code added to WP Rig.