Renamed hack-nights to events and (mostly) related cleanup

[drubgrubby]

Fixes #748

This main part of this was renaming hack-nights.html to events.html.

In addition, there was some cleanup to related files:

Add underscores to .scss files for consistancy
Remove undescore from _main.scss
(cnk suggested the above two changes to keep naming convention consistent)
Change hack-nights-page.scss to _events.scss
Delete ../website/_includes/hack-nights.html
Moved a couple of styles from hack-nights.scss to events.scss
Added events.scss to main.scss

[KianBadie]

@cnk asked me to do a review visually on this. I did a pretty shallow visual review because I did a pretty thorough-ish review of the css classes and came to the conclusion that much shouldn't change visually because most of the classes were unique to the page on not used anywhere else. Everything looks good to me 👍

However (not because of your work @drubgrubby but I think these notes stem from things that were already existing on files related to the page):

• .meetings-message was used/defined in project-meetings/html.scss, however they both use the same exact rules so no problem there
• active::after is used in 2FA guide page and getting-started-page.html/scss, however the rules on the events page just reapply the same exact rules to mobile, so no problem there either
• I know .section-hack-nights and .btn got moved to events.scss, however it is effectively imported in the same order, as events.scss is imported where these classes used to be imported. So no problem there as well as far as I can tell.

@cnk @DanielekPark I did notice a bug (that I'm pretty sure was inherited and not from your work @drubgrubby), but because active::after is defined in getting-started-page.scss for desktop views, it applies to this page as well. Which means, if you click the "Meeting Events" header under "Our Events", it will add a drop down arrow to the left of it. Click it again and it makes the content disappear. This is already on the main site, so again, it's not from this work.

Good work @drubgrubby