Add integration tests with testify/suite. Moved some structs
outside of controller funcs so we can re-use them in the testing
package. Add CI workflow for running tests on every push.
* remove cmd folder as we are going to have only one entrypoint
* get rid of pkg directory
* rename test -> integration_tests as unit tests should reside next to the actual files they are testing
* database migration WIP
* reinstate gorm boilerplate in the addinvoice for now to make it compile
* introduce migrations
* add Makefile
* don't use unsigned types for database mappings
* migrations work now
* add build target
* use echo groups
* gorm removed
* add envconfig
* fix comments
