Split WorkflowLoader into multiple classes

This patch doesn't add any new code, it just spreads out the code from
WorkflowLoader into the existing WorkflowLoaderFactory and two new
classes BlockFactory and SubmissionHandler. The API's for 
WorkflowLoader and WorkflowLoaderFactory are not changed so no other
code requires update.

While trying to figure out how to test blocked users going through
WorkflowLoader I realized the loader does too many disparate things.

This patch splits it into discrete pieces that better match SRP.  In
addition previously WorkflowLoader was doing a fair bit of work in the
constructor, going all the way to the database.  Constructors should
have minimal setup so moved that into WorkflowLoaderFactory

At this point WorkflowLoader is now just a shell, consider further
refactors.

Note that current intention is to change the forms to use the same
parameters as API requests expect, and change the backend page rendering
process such that it makes an internal API call and passes those api results
to lightncandy.  At that point it may make sense to delete the WorkflowLoader
and use the individual pieces within FlowApiBase or some such.

This was started against frontend-rewrite branch, but is now against
the master branch.
Original gerrit change: I22a36a55e9b4704932e93d2f117a79fa1ea146a6

Change-Id: Id38a93bed37fcf8b91b815121f7c0537f25a525e