Add Stripe Plans

[pbrisbin]

• Add Plan model and Plans route
• Add Handlers to Purchase or Cancel
• Migrate Personal and Business plan records

[jferris]

This looks good to me. I had one minor comment, but I think you can merge this whenever Travis cheers up.

[JoelQ]

LGTM once CI is green :rocket:

[pbrisbin]

So. Here's what seemed to be happening with the builds:

• Travis was flaking due to out of memory errors
• We tried a Circle build from a different branch to see if it had the same issues (so far it doesn't)
• I tried a Circle build on this branch, and got Green
• Now that Travis can use a cached sandbox (built successfully on Circle), it doesn't fail for out of memory but does encounter a shared library error because the sandbox built on Circle is not compatible with Travis in a very subtle way (I don't know all the details here, but can dig/explain if we think that's valuable -- I do not).

I suggest the following:

• Move this project to CircleCI: cherry-pick the circle.yml into master (and remove travis.yml)
• Rebase, Green, Merge, Deploy