semantic markup for main events listing

Notably made each month’s list of events an unordered list. Each month
is its own section with aria-labelledby referencing the id of the
child h2. That id is the name of the month, which should be fine as
long as we don’t list events more than 12 months in advance.
Also cleaned up extraneous attributes on main.
We should probably add maps to the `gitignore`…