perf - move stripNotes to after we know if it's needed

[cscheid]

This is a bigger one that I found - we're running a fairly expensive check inside every div and span for Note nodes. This PR moves the check so it happens after flags, and then we know whether the document contains footnotes at all.

{'outcome': 'significant', 'winner': 'B', 'p_value': np.float64(1.3498071778535291e-06), 'total_samples': 20, 'alpha_spent': 0.05, 'statistics': {'counts': {'A': 10, 'B': 10}, 'means': {'A': 0.776877474784851, 'B': 0.7512164831161499}, 'variances': {'A': 7.183128859722387e-05, 'B': 2.541789535892569e-05}}}
Statistical Summary:
===================
Variant     Count         Mean      Std Dev
-------- -------- ------------ ------------
A              10     0.776877     0.008475
B              10     0.751216     0.005042