Background URL Handling

As a user, I want to know when a background URL I have provided is accepted or invalid, so I am aware whether the link I supplied will work with Mynt Dialer.

[x] When a background URL has been updated successfully, use a PopUpModal to notify of the successful operation.
[x] If a background URL returns an error, prevent erasure of the previous background URL (don't change from current settings) and then use a PopUpModal to notify the user that the supplied link is not valid.

SkylerBurger / new_tab_dialer

Background URL Handling #72