Pull request page allows interaction before it is ready (BB-10345)

More specifically the Merge button's functionality only becomes available much later in the page load.

On my connection(250ms latency) the page renders typically in 2-4 seconds and continues to load for another 15-25 seconds. (Total page load is +- 1.2MB and takes 20-30 secs).

If I press the merge button after its rendered but before the page is fully loaded then the browsers loads and displays the JSON result from the /fulfill ajax request instead of the merge dialog.

As such I have to wait until the page loads completely for the Merge button to function and no indication of this is given resulting in me having to reload the page when I clicked the Merge button too early(Within the 15-25 secs of page loading after render).