Packaging of frontend assets

Since the inception of Heptapod, we've had no consistent way of packaging changes in frontend assets, because these are precompiled in the Debian package on which the upstream Docker image is based. This includes notably various images and JavaScript code. Only source installations could take advantage of frontend changes, and obviously we didn't want to create differences between installation types.

This currently blocks many features (all issues bearing the Front label`). Also, with many UI GitLab elements being rendered in JavaScript, this problem is getting automatically worse over time.

Here's a plan to overcome that in the short run:

• omnibus-heptapod#1 (closed): create packages including Heptapod Rails app sources instead of GitLab Rails but otherwise identical to the upstream package.
• omnibus-heptapod#2 (closed): new intermediate base Heptapod Docker image, based on the package produced by previous step.
• heptapod-docker#10 (closed): use this new Docker base image instead of the upstream GitLab image.

(writing this on the main project instead of heptapod-docker because the involvement of the latter will be minimal and the visible consequences are very much about the Web application)