For abelian fields, only compute character group if the conductor is small enough or the page may take too long/time out.
For a quadratic field, compute its Dirichlet character directly for speed.
Use conductor cutoff for computing Dirichlet characters of quadratic fields.  We can compute the characters, but the links would be useless.
Merge pull request #169 from jwj61/master

Conductor bound for computing Dirichlet characters of an abelian field
Merge branch 'master' of https://github.com/LMFDB/lmfdb
Now show error message when can't tensor product.
Better tests for tensor product pages.