When validating repositories, we build a list of subforms that we're

operating on and check them for data or errors. This is done in two

locations, depending on whether the form has posted data, and some

upcoming work for SCMTool-provided forms would have doubled this logic.
This is ultimately pretty error-prone, and hid a bug where, if you

choose a bug tracker hosting service and then enable the "Use hosting

service's bug tracker" checkbox, the bug tracker form was still being

populated with data.
We now loop through all the bound subforms and use those, instead of

manually building a list of subforms. This is done through an

iter_subforms() method, which takes criteria on which subforms should

be returned.
Unit tests take advantage of this method to check that we always work

with exactly the subforms we expect, so others don't slip through the

cracks.
It also fixes up the is_valid() logic, which was duplicating a check

unnecessarily for a valid subform. That now relies entirely on the

subforms_valid state we already calculate.
This will help keep things maintainable as we add SCMTool-specific

subforms. Note that we also aren't including authentication subforms by

default at this time, since they're treated pretty differently, but

we're now in a better position to change that going forward.

Unit tests pass.
Tested saving repositories for different hosting services and for

standalone repositories.
Tested saving bug tracker information in all bug tracker modes

(no bug tracker, custom bug tracker URL, hosting service bug tracker,

and with the "Use hosting service's bug tracker.")