Themes: Refactor themes-options

[ockham]

Kill, kill, kill for peace.

Addresses #1620.

Some ideas:

• Ideally, the getButtonOptions call (or equivalent) would not happen until the ​_more_​ button is clicked ((c) by @seear)
• Reduce arguments to getButtonOptions
  • Don't pass actions. Instead, just return the Redux action for further dispatching by the consumer
  • Don't pass theme, site, and isLoggedOut. Instead, make isHidden a function accepting those directly.
• For clarity's sake, return an object from getButtonOptions (with what's currently the name as the key) instead of an array (I have code locally for that)

[ockham]

Closing in favor of #2378.