Make packages are available at "/packages"

This MR is bades on the previous one !12 (merged). It should be merged before.

Implemented as written in https://gitlab.com/fdroid/fdroid-website/issues/11#note_20964160.

See

• https://nicoalt.gitlab.io/fdroid-website/packages
• https://nicoalt.gitlab.io/fdroid-website/packages/apps/org.fdroid.fdroid

Related to #11 (closed).

CC @eighthave