Add database schema

[jmaupetit]

Purpose

Now that the API can receive static and dynamic data, we need to save them!

Proposal

• [x] fix Static.id_pdc_itinerance (should be required)
• [x] add static schema database models
• [x] make all database models timestamped (add created_at, updated_at field)
• [x] add database migration
• [x] run database migrations in the CI
• [x] run database migrations in staging
• [x] install / enable postgis extension in development
• [x] install / enable postgis extension in the CI
• [x] enable postgis extension in staging environment
• [x] update the CHANGELOG
• [x] improve Localisation.coordonneesXY de/serialization