Add credits page

[jasonaowen]

Add a page that includes the licenses of all of our dependencies (as managed by gradle and, someday, npm - #70). Those licenses should be automatically gathered and the page(s) generated at build time.

We should able to add credits for vendored assets, like images licensed under a Creative Commons license that allows for reuse. (So far, the only image in the repo is the user placeholder, which was licensed CC0; I chose that image because of the license so I didn't have to set this up yet!)

Ideally, our vendored assets would be credited in a Markdown file in the root of the repo, which would be rendered as part of the build process into HTML that can be served from the site.

[jasonaowen]

Consider the dependency-license-report Gradle plugin; I found the report it generated to be good, but I didn't spend enough time to get that output into the application.

[jasonaowen]

See also the best practices for attribution on the Creative Commons wiki.