fix(oauth): increase the assertion lifetime to avoid clock skew issues
Merge pull request #1655 from mozilla/issue-1649-assertion-expiration

fix(oauth): increase the assertion lifetime to avoid clock skew issues

Since the leaf portion of the assertion has the exp time set by the client,
a client with a very "off" clock can produce assertions that are already expired.
Making the exp time "a long time in the future" effectively makes the lifetime
of the assertion  determined by the lifetime of the signed public key
(currently 6hrs), which is an acceptable tradeoff.

r=ckarlof
Fixes #1649