Improve interface translation fallback if Update module is disabled

Problem/Motivation

Update of interface translations has a soft dependency on Update module. Update module is required to determine a stable versions in case a dev-release module is installed. Translation packages are only available per release, not for dev-builds. The fallback when Update module is not installed, is quick and dirty. One of its side effects is that no interface translation status is available (/admin/reports/translations).

Proposed resolution

Use the new download URL that was added in #2113957: Build server side version fallback system for translations from which the translation file of the latest stable release can be downloaded.

An issue has been created to provide better user feedback (#2501035: Without "Update Manager" module enabled, "Available translation updates" does not get updated, should be documented), but it is better to fix the cause than the symptoms.

Remaining tasks

• make patch
• write tests

User interface changes

none

API changes

none

Data model changes

none