Re-implement "improve this content button"

Since https://github.com/prose/prose/issues/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 :heart:'s user-friendly experiences, even for the uninitiated.*