Make sticky header optional

Adds an option to Theme Options > Basic Settings to hide the sticky header, but will by default show it. If the sticky header option is unchecked, the checkbox option to show the site name in the sticky header will also disappear, toggled via jQuery like other options here.

If the sticky header is hidden:
- The global nav in partials/nav-global.php will display
- The main nav in partials/nav-main.php will display

For #247.