Update design to be visually consistent with CfA's other web properties

[tdooner]

The Brigade website uses an (ancient) version of the style guide which we no longer support. This results in a web presence that is visually inconsistent, and generally not as refined as our refined volunteers deserve.

We are migrating to "v4" of the style guide, at v4.style.codeforamerica.org. Work is progressing in the feature/style-guide-v4 branch.

Here is a TODO list of things before we declare the refresh complete:

• The alerts in the style guide need to be updated
• We need to figure out what to do with the header links that scroll to a section later in the page
• The project showcase page needs significant work
• Add scss-lint based on GCF config with BEM and alphabetized properties.
• The project finder page needs significant work
• The brigade page needs significant work
• Add mobile nav menu
• Audit styles for header hierarchy to match style guide. (Remove h2 font weight)

[tdooner]

We got it done!