Re-purpose this repo for popcorn-editor

[cadecairos]

popcorn-editor is the future of Popcorn Maker. As such, we shall take the current contents of https://github.com/mozilla/popcorn-editor/tree/master and make that the current master branch here.

TODO:

• [X] Create a popcorn-editor branch, tracking https://github.com/mozilla/popcorn-editor/tree/master
• [x] Clean out Issues in this repo
• [ ] Modify the README in popcorn editor to explain the change from popcorn.webmaker.org - or link to a blog post doing just that. Also link to the last stable release tag of popcorn maker - v2.1.41
• [ ] replace master branch with popcorn-editor branch
• [ ] Delete mozilla/popcorn-editor - Migrate issues here?
• [ ] Rename this repo popcorn-editor
• [ ] rejoice

[cadecairos]

@Nolski @benrito