fix Case:58847 (adding a question takes painfully long on large forms)

Currently when you add a question on a large form it can take several
seconds.  This is because every time a question is added every single
question in the form is validated in order to redraw the error icons
in the question tree.

This change removes that and makes adding a question snappy on even very
large forms.

Revalidating all questions seems unnecessary because this is already
done on form load and when you change a question that question itself is
automatically validated.

Since by default questions are created in a valid state (non-duplicate
question id, etc), adding a new question never introduces new validation
errors until the question is edited to be invalid, and anyway if the
question wasn't created in a valid state you would see an error as soon
as you edited it.

As far as I can tell validation only occurs isolated to each individual
question and there is no structural validation except for a few things
like unique question IDS.
Merge pull request #55 from dimagi/case-58847

fix Case:58847 (adding a question takes painfully long on large forms)