Add a system for upgrading accounts using upgrade codes. Users purchase
an upgrade code off-site then redeem it on-site to upgrade their account
to Gold. Upgrade codes are randomly pre-generated and are one time use
only. Codes have enough randomness that guessing a code is infeasible.
* Remove .ui-corner-all, .ui-state-highlight, .ui-state-error classes
from notice boxes.
* Use .notice, .notice-info, .notice-error classes instead.
* Replace <p> elements in notices with <div>'s so that we don't have to
work around the `margin-bottom: 1em` from <p> elements.
* Replace <h1> elements in notices with <h2>.
* Standardize info notices to use the same shade of light yellow in the
light theme.
