semantic markup for main events listing [revised]

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>.
[This is updated commit using camelCase for class/id names, and better
SASS syntax for James. Also added styles.css.map to gitignore as per
Pete’s sugg.]