Fix AJAX spinner by assetizing relevant files.

In https://github.com/prometheus/promdash/commit/32fbebbbcdaf141cb75f6e67af7b48149c03737c, the reference to `spinner.gif` was changed from `<%= image_tag("spinner.gif", alt: "AJAX indicator") %>` to the literal `/assets/spinner.gif` because the graphing widget was broken out into its own (non-erb) template. This broke `spinner.gif` URLs in a production deployment, since the GIF link was then missing the asset fingerprint.

This PR is one way to fix this: it moves the template and referencing JS files back into the asset pipeline and uses the dynamic asset linking methods again. Another possibility would be to simply move the `spinner.gif` out of the asset pipeline and put it into `/public`. Not sure which one is better. Thoughts?