don't store signatures in notebooks

store them in an sqlite database instead.

The algorithm and signature are stored and not associated with a path.
This means that moving files around doesn't affect trust,
and every trusted change to a notebook remains trusted.
automatically cull oldest 25% of signatures

when cache size is reached
remove signatures from example notebook metadata
add counter to new notebooks in test_manager

ensures brand new notebooks are untrusted

previously, all new notebooks were identical,
so once one was trusted, they would all start trusted.
clear signature db on reset
Merge pull request #7244 from minrk/rm-signature

don't store signatures in notebooks