Re-implement "improve this content button"

[benbalter]

Since prose/prose#643 has remained unresolved since early December, this pull request swaps out the Prose-based "Help Improve this content" button, for one that links directly to GitHub's editing interface.

Although admittedly not ideal, I'd argue that a working button that allows the user to complete their designated task is superior to a button that doesn't work but has a more user-friendly interface.

The flow for authenticated users will now be:

1. Click "improve button"
2. Project will be forked, if not already
3. User makes proposed changes
4. User clicks "propose change"
5. Pull request is created and user is prompted to provide additional details

The flow for unauthenticated users

User will be asked to login to or signup for GitHub, and then flow will be the same as above.

Fixes #315, /cc @lukefretwell, @leahbannon

Full Disclosure: I'm a GitHub employee who hates bugs and ❤️'s user-friendly experiences, even for the uninitiated.

[leahbannon]

👍

[konklone]

👍 Editing's gotta work.

And Prose's UX for editing, while comforting and easier in some ways, has been stagnant for a long time (as has the project generally). At this time, it's not a great horse to tie the project to.

[gbinal]

Great edit. Thanks!

[haleyvandyck]

Thanks for helping fix this @benbalter. If/when the prose bugs get addressed we can reassess then, but for now this seems like the best direction to move...Thanks for the PR.