Create shared Glossary between all Element versions (Web, Android, iOS)

At now translate.element.io contains separate glossaries for each Element version (Web, Android, iOS). As result, same source phrases very often translated differently in each application, that makes understanding harder.

Manually syncing content of all glossaries is too expensive task, much easier is make one "main" glossary, shared between all Element versions, and give to users selection to which glossary add phrase - to "Element Anrdoid" only, or to "All Element versions".

And maybe better to name new shared glossary as "Matrix clients" or even "Matrix", for ability to reuse same glossary between all current Matrix clients (Nio, Ditto, etc), and new clients, also use it as base for other projects.

Start at translate.element.io and inspect how the separate Element Web, Android, and iOS glossaries are configured and selected. Clarify whether the work belongs in this repository or the translation service, then define the shared-glossary scope and a way to choose between a client-specific glossary and one shared across versions. Done means the ownership, configuration changes, and synchronization or reuse behavior are documented and agreed.

Written by the indexing model from the issue text.