Use display version in update process

[pjrobertson]

Fixes #3048

Also:

1. Uses https://github.com/quicksilver/Quicksilver/releases/ for the changelog URL going forwards
2. Updated the website to redirect to this page
3. Removed all references to the old changes/changebare stuff.

[n8henrie]

Did you manually populate (i.e. type into the website) the changelog at https://github.com/quicksilver/Quicksilver/releases ?

[pjrobertson]

Previously, the flow was:

1. Manually write into the CHANGELOG file in the repo, commit and push.
2. During the build, this gets converted to HTML (python markdown) and added inside the app.
3. During the CI release this file also gets pushed to the qsapp.com website.

I'm now proposing (have implemented):

1. Have Github automatically generate the release notes for each release (which it does pretty well)
2. Redirecting all changelog links to the GH /releases page (already done)
3. Redirecting clicks from within the app to the GH /releases page (already done)

So going forwards, https://github.com/quicksilver/Quicksilver/releases will be our single place for release info. It'll be easier to edit/update and maintain, and also includes downloads right with the release notes.