Add support for various licenses to be added to individual posts

[crutchcorn]

• Define a structure for LICENCE.md to be placed in a file (or a frontmatter field?)
• If not using frontmater field, add the LICENCE.md to the GraphQL calls for Unicorn Utterances
• Provide a default licence? (@fennifith what do you think? @evelynhathaway do you know of any reason legally we couldn't/shouldn't do this? I don't know if we should, seeing as one could accidentally commit a post and that way we can CI to force a check for a LICENCE.md file and remove legal ambiguity) NOT DOING, SEE BELOW
• Mention the licence layout in README.md
• Include an image of that licence in the bottom or have a sidebar with info @tommyemo would love some thoughts on a design for this. I don't know what would look better or where things would go

[fennifith]

I don't think a default license should be provided, tbh. We can have a "primary license" or suggest a few of them in the README and have pull request templates for contributors to choose from, but IMO having default licenses would make it too easy to publish something "by mistake", as you said.

[crutchcorn]

I like having a primary licence, suggest them (and probably point to something like https://tldrlegal.com/), and provide a PR template would all be solid options that I like significantly more than having a default license

[crutchcorn]

We have initial work on this completed. I have further questions about what we want to do in the future but it seems out-of-scope.

Closing this in favor of: #35