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.
Sorry, I'm going to be anal here and ask you to change a couple of things:
 - Can we use camel case like the rest of the classes (`monthEvents`)
 - Can we nest the ul inside the month events class (will output the same, just keeps things within context in the sass file)