Use moz-toggle in connection preferences

Our connection preferences currently use <checkbox class="toggle-button"> but next ESR release will contain the <moz-toggle> (bugzilla bug 1799466) which we can use. Basically, we want to follow bugzilla bug 1799471.

Note that it changes the accessible role of the element from a "checkbox" to a "toggle-button".

The main difference in firefox is that they put their <label> before the toggle. E.g. see the firefox quick suggestion Address Bar settings when "browser.urlbar.quicksuggest.enabled" is true. Similarly, the addon toggle tends to be end-aligned. We could optionally follow the same style, but I think we'll be ok just placing the <label> manually ourselves.