Versions of extensions

[christianp]

I've rewritten an extension, so questions written for the old version will no longer work. I also regularly have to use my local server to test new changes to an extension, so I don't inadvertently break other people's questions while debugging.

The editor should store multiple versions of extensions, and references to extensions should include the version. Declaring a new version should be an explicit step. There should be the option to always use the "development" version, but this should not be the default.

[chrismgraham]

Good idea. For the extensions that sit on Github, should/could the versions be linked to github versions/branches?

[christianp]

that's related to #638