Set maximum_discourse_version and add end-of-life info

[davidtaylorhq]

This will stop the theme component from causing catastrophic errors when installed on up-to-date Discourse sites.

3.2.0.beta4 is the last version which shipped with Ember 3 by default.

[angusmcleod]

Much appreciated David. I would have loved to have saved this, but it's not possible time-wise.