Add Slack integration: active users and invites

Resolves #79

Previously, there was no information on the Code for Tucson website concerning our Slack team.

This PR adds a new Slack integration via slackin on the front page and also via an SVG icon on the GitHub README.

Before screen shot 2016-07-20 at 12 56 10 am

After screen shot 2016-07-20 at 12 55 53 am

with invite dialog open screen shot 2016-07-20 at 12 55 57 am

Notes

This introduces some new JS based on an external service. Currently it's running on a personal Heroku instance (free) and we can easily add some other C4T members to that app. If that app were to crash for some reason, however, this button simply wouldn't load. I do not believe that there would be any indication in the browser that things failed - it should rather simply fail to appear.
I added an SVG icon to the README for fun because it's there and we can, but we don't have to if we don't want to :wink:

cc: @meiqimichelle

CodeForTucson / codefortucson-site