API: Introduce "templated parameters"

With MCR coming up, ApiEditPage is going to need to be able to take
"text" and "contentmodel" parameters for each slot-role, and enumerating
such parameters for every possible slot would probably get rather
confusing as to what is required when, or at least long-winded in
repeating the exact same thing for every possible role.

So let's abstract it: we'll have an "editroles" parameter to specify which
slots are being edited, and ApiEditPage will just declare that
"text-{role}" and "contentmodel-{role}" parameters should exist for each
value of "editroles" in the submission.

Note this patch doesn't introduce anything that uses templated
parameters, just the functionality itself. For testing purposes you
might cherry pick I2d658e9a.

Bug: T174032
Change-Id: Ia19a1617b73067bfb1f0f16ccc57d471778b7361