Add Slack invite button

[dmsnell]

Related to #65

We don't currently have a smooth system in place for inviting new guests into our Slack channel. Some brigade websites have a Slack invite button on their front page, which presumably would send an email or notification to the Slack admins.

I think that we could benefit from incorporating this into our site.

cc: @meiqimichelle

[meiqimichelle]

I believe the only way to get an auto-invite to Slack is to host an app somewhere. However, there might be other options now that didn't exist the last I checked. Here's a quick google scan:

• InviteBot
• SlackStorm

If someone can sort out the backend logistics of such a thing, I would love to add it to the site.

[dmsnell]

@meiqimichelle I can look into this and plan on doing it later. My guess is that it shouldn't be too bad, and even if it requires something, we might be able to get it for free as an organization.

Helpful would be to have some advice on where to put such a thing.

[meiqimichelle]

@dmsnell I'll start work on #63, should help sort out where it might go.

[meiqimichelle]

As a quick-fix tho -- we can add something under this part of the current site:

[dmsnell]

Lol, the Slack button featured there on the Atlanta brigade is the product of a former Automattician and awesome programmer, Guillermo Rauch.

It does indeed run on an app host somewhere, and I think that it might be worthwhile. In any case, I believe that we can get a single Heroku instance running for free.

It's an iframe that embeds in the website; will try and pull together a PR.