Add labels to the metadata in user files

[scizzorz]

All lines in a user file should be prefixed with the type of information on that line. The goal is to make reading and updating user files a little easier for the humans involved, in addition to making them a little more robust and powerful for the parser.

Example:

email jweacho1@binghamton.edu
section a0
grade assignment01 10 10
grade lab01 10 10

Note: there should be a space between the label and the data, but there should be tabs in the grade values.

Also fix a bug where editing your email would erase all of your grades, because user.write() never wrote grades back into the file.

Migration

Hopefully this should update all existing user files: migrate.py

[philipdexter]

Also fix a bug where editing your email would erase all of your grades, because user.write() never wrote grades back into the file.

I knew I forgot something... nice catch.