Redesign front page and move its code into its own file

[leeklarich]

I'm adding a second box for the slack link next to the sign up box on the home page. I will also format the sign up box to have the "sign up here" label inside of the box.

[leeklarich]

I'm moving towards the new redesign provided by Linda, and I'm going to move the code from the header file into the home page file.