Make it easier to add/remove buttons from the "Donate" area in the sticky and global nav

[benlk]

Parts of this:

• remove the hardcoded donate button
• remove the donate button options from the theme settings
• create a menu that is output in this location, where arbitrary links can be added like "Donate" and "Subscribe'

We've had to make lots of customizations to nav partials; this would remove the necessity of making those edits.

[benlk]

Note that the Largo Donate Widget doesn't require this URL to be set to be used, so it doesn't block this issue.