Add a form to set priority of top 2 providers

Their priority should always add up to 100%. Currently we have to ensure
this by hand. Adding this form means there’s no way to not set their
combined priorities to 100%. And it’s a bit more of an intuitive UI than
two textboxes on separate pages.

gulpfile.js

@@ -159,6 +159,7 @@ const javascripts = () => {
     paths.src + 'javascripts/colourPreview.js',
     paths.src + 'javascripts/templateFolderForm.js',
     paths.src + 'javascripts/collapsibleCheckboxes.js',
+    paths.src + 'javascripts/radioSlider.js',
     paths.src + 'javascripts/main.js'
   ])
   .pipe(plugins.prettyerror())