Create github releases for all past GWT releases

[niloc132]

Past Google-created GWT releases used goo.gl and Google storage API for the SDK zips.

Following the news of goo.gl links being deleted (see gwtproject/gwt-site#362), we should re-upload copies of past releases and make them available via the github release page instead, to defend against the eventual deprecation/removal of that service.

[jnehlmeier]

Maybe also upload the legacy dev mode plugins to github and update the download links at http://www.gwtproject.org/missing-plugin/ ?

[niloc132]

Great thought, especially since Google never provided build steps for the plugins that we do have...