UX Improvements to the Media Dialog

In this fix:
* Adding 'go back' button to appear in the search panel.
* Hiding/showing the correct buttons within each panel.
* Disable the 'apply' button, and only enable when changes are done.
* Load the search query API and send API requests only when the
  search panel is up.
* Make sure the search query is updated with the page name every
  time the search page is loaded

Bug: 67443
Change-Id: I635986f30579c6358e0cc6fe54fbf97f49c83465
Followup 26fd3777: hide "Go back" in insertion mode

"Go back" should only be shown if there's something to go back to.
So it should be visible if you came from "Change image", but not
if you came from "Insert media".

If the "Go back" button is hidden, the footer is empty, so hide it
completely in that case. Hiding the footer is tricky due to how
dialogs position things, so I hacked around OO.ui.Dialog's lack
of an API for toggling the visibility of the footer.

Change-Id: Ic97c4a2bf7beaa240e5a4dfda1e605752e1f4189
Merge "UX Improvements to the Media Dialog"
Merge "Followup 26fd3777: hide "Go back" in insertion mode"