Responsive styles - frontend announcement list

[cjg89]

• Main site navigation needs some extra breathing room between it and the Announcements h1. We've got main site v3 set up to use a dark gray navbar on pages without a header image--will post a link in Slack
• Consider breaking out of the masonry layout at the -md breakpoint and below for announcement card contents to not clip out of their containers
  
• Consider using a smaller font size for the Ongoing Announcements heading; make sure it does not cause horizontal scrollbars at smaller screen sizes
• Consider removing the "Learn More" buttons on each announcement card with a link wrapped around each announcement title
• Consider using a smaller font size for announcement titles, and possibly for audience and contact info

[jmbarne3]

Closed by #13