Challenge signup search

RELEASE NOTES: Challenge moderators can now search the signups for their challenge by pseud instead of having to manually find them in the alphabetic list, which can be very long for a large challenge. 

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https://code.google.com/p/otwarchive/issues/detail?id=2867

Signups for a large gift exchange should now be searchable by pseud. Has tests. 

Also fixes a tiny unclosed &lt;ul> tag on the prompt meme signup. 
Hi! When you update this pull request, could you change this to "Sign-up" like the other instances of the word? Also, the title attribute on line 13 and the submit button text on line 14 should be `ts()`-ified, I think.