Get homepage working with username; #287, #680
Rename %participant_id dir to %username; #287
Add long id to security.txt, ht @dstufft
Bump version to 7.3.31
Bump version to 7.3.31-dev
Merge branch 'master' into username
Big changeover from id to username; #287

I sort of did this one with the blast shield down. I haven't even run
the tests yet. The sorts of things I changed:

 - SQL {elsewhere,exchanges}.participant_id => .participant
 - SQL participants.id => .username
 - ORM {user,participant,self}.id => .username
Started working through test failures; #287