Add a Loading paragraph onto zuul's status page

When watching Zuul status' page on networks that are not completly
reliable, it can take up to 20 seconds for the status page to show the
queues, especially if the JSON blob to download is large. So now the
HTML page adds a 'Loading...' paragraph into the div, which the JS will
very handily remove when it parses and adds the JSON into the DOM.

Change-Id: I2519300c2e9323bfd9379b792832e5ffef027f26
Merge "Add a Loading paragraph onto zuul's status page"