Add past ADRs #15

We adopted many Architecture Decisions before using ADRs, and they should be added to this repository as it acts also as a documentation of past acts.

ADRs to be written retroactively:

• Symfony migrated page use the Grid for listing
• The Grid rely on Doctrine, not ObjectModel
• The use of CQRS
• Configuration Forms and Identifiable Object Forms
• Using and extending Twig
• Mail Templates
• The use of CLDR
• The use of UI Kit
• Why some modules are in the ZIP, packaged by Composer, why some module are not
• Testing tools, conventions, strategy