Rework compilation to no longer require active database

[Victov]

sqlx has some option to generate a json file to be able to compile your project without requiring an active database connection.

This will improve iteration time, simplify the github actions and lower the barrier for people to get working on the project.

• Get sqlx to generate that json file
• Enable database-less compilation
• Update github actions to work with the json file instead of booting and seeding a database
• Update readme
• Compile-time checks that the schema still matches the database must remain intact, do not change the rust code so that the compile time checks disappear.

Azerust has this setup working, for inspiration

[Victov]

Taken care of in #55 and updated github checks in #56.

Offline compilation is now possible, given that the .env files are properly setup. .env.template file have been updated, so copying those should do the trick