R1:2b538bfb2575

Use classes to define standard field types and implement an "int" type

Summary:
Currently, `ManiphestAuxiliaryFieldDefaultSpecification` uses about a dozen giant `switch` statements to implement stadard field types (int, string, date, bool, select, user, remarkup, etc). This is:

- pretty gross;
- not extensible; and
- doesn't really let us share that much code.

I got about halfway through porting a similar implementation into StandardField but I wasn't thrilled with it. Subclass StandardField…