Switch from uiModal to uiConfirm, adjust styles

A few things going on in this commit:
- uiConfirm already has an OK button, so I'm trying to use that instead of uiModal.
- The confirm OK button cancels, which is maybe a problem. I might change it.
- Dispatch a change event instead of trying to call a function back in uiBackground
- Remove some of the custom css, since we already have some shared button css
- add utilNoAuto to textarea (this just prevents autocomplete and other
  annoying behaviors especially in Safari)

data/core.yaml

@@ -515,7 +515,7 @@ en:
   settings:
     custom_background:
       tooltip: Edit custom background
-      heading: Custom Background Settings
+      header: Custom Background Settings
       instructions: "Enter a tile URL template. Valid tokens are:\n   - {zoom} or {z}, {x}, {y} for Z/X/Y tile scheme\n   - {-y} or {ty} for flipped TMS-style Y coordinates\n   - {u} for quadtile scheme\n   - {switch:a,b,c} for DNS server multiplexing\n\nExample:\n{example}"
   restore:
     heading: You have unsaved changes