R1:20e4c3fbd46b

Modularize complex HTTP parameter types

Summary:
Ref T9132. We have several places in the code that sometimes need to parse complex types. For example, we accept all of these in ApplicationSearch and now in ApplicationEditor:

> /?subscribers=cat,dog
> /?subscribers=PHID-USER-1111
> /?subscribers[]=cat&subscribers[]=PHID-USER-2222

..etc. The logic to parse this stuff isn't too complex, but it isn't trivial either.

Right now it lives in some odd places. Notably, `PhabricatorApplicationSearchEngine` has…