Improve middle initial field for registrant

[scotthue]

Related to #343. Right now the middle initial field in the new registrant form allows 255 characters upon entry, but it is only ever displayed as middle initial. Additionally, there is no label on the field and thus users don't know what it is for.

Example issue:

This user should have their name displayed as "van der Staak, Marieke" but input her name in incorrectly due to the confusing fields.

A very simple solution would be to limit the field to only allow 1 character upon input. And perhaps updating the DB to make all existing registrant data truncate that data to 1 character to prevent errors.