Compare emails without case when reopening forms

Although the standards say that email local-parts can theoretically be case-sensitive, I think in practice we're vanishingly unlikely to have two people whose email addresses differ only in this respect, whereas people varying the capitalization when writing down case-insensitive email addresses definitely happens and has led to problems matching registration forms.

This change updates the code to compare email addresses in a case-insensitive way when checking for existing forms.

Closes #76 (closed)