The admin change form pages were heavily reliant on there being a model

that was being modified. We were basing the form IDs on this entirely.

We did allow the ID to be customized through a form_id block, but that

only worked for the <form> tag. The attribute was not passed to the

underlying PageView class. Since those are in entirely separate blocks

on the page, we weren't able to really define the variable in any good

place.
Now that Djblets and Review Board support definition of global variables

in templates, we can address this. This change defines the form_id

variable as a new global variable in the new global-vars block,

utilizing the form_id block. We can then reference this both for the

form and the JavaScript.
This fixes a handful of issues with the settings pages, and other pages

using the change_form.html template.

Tested several pages using this template. Verified that the form ID was

correct in each case, both those that used a model-based ID and those

that overrode the block.