Make formpost work with multiple tempurl keys.

Formpost wouldn't look at X-Account-Meta-Temp-URL-Key-2 for signature
validation, which was confusing to users. Now it does, and there's a
utility function that formpost and tempurl both use to help prevent
divergence in the future.

Also removed formpost's custom cache-lookup logic. Formpost was
looking for tempurl keys in memcache, but it's been broken since
commit 21343ab (2013-05-02) when tempurl changed its memcache keys, so
the lookup would always miss. (Since tempurl started storing *lists*
of keys, formpost would have choked on the new data, so that oversight
was the only thing keeping formpost working at all.)

Now formpost just uses the same get_account_info() call as everyone
else does, so it gets all the benefits of the new caching framework.

Change-Id: I426d8f2882ebe3ce373de604b9ff18846f5999e3
Merge "Make formpost work with multiple tempurl keys."