Allow viewport scaling (zooming) of frontend (#7180)

The inclusion of the `user-scalable=no` value in the viewport meta tag
prevented viewport scaling, disabling the ability to zoom the webpage.
This most typically affects mobile devices, given the nature of the
`<meta name="viewport">` tag.

Removing the restriction allows a user to zoom in to see small and fine
detail in the UI -- such as zooming in on particular areas of a home
security camera streams or other images, inspecting detail in state and
other graphs, and so on.

For users with accessibility requirements, such as low vision
conditions, being able to zoom the frontend means they can enlarge UI
elements to suit them (MDN explains several accessibility concerns at
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta/name#Accessibility_concerns_with_viewport_scaling)

This change has no effect on users that choose not to use it (for
example, only those that engage zooming such as via pinch-to-zoom on
mobile devices will see the change) -- the frontend remains the same
otherwise.  Elements of the frontend that do use pinch-to-zoom (e.g. the
Map) continue to work as expected, with pinches on that screen area
being captured by the map.

src/html/_style_base.html.template

@@ -1,4 +1,4 @@
-<meta name='viewport' content='width=device-width, user-scalable=no, viewport-fit=cover'>
+<meta name='viewport' content='width=device-width, viewport-fit=cover'>
 <style>
   body {
     font-family: Roboto, sans-serif;