Migrated Pless to multi-project structure.
Pless is now part of an aggregator project. Tests run correctly (even with Jacoco, but that had to be fixed...). Development mode also runs.
Started moving test classes to plessTest.
Moved test helpers for setting up test environment: email providers, client session storage, factories, and services.
Moved session-related classes to the `si.urbas.pless.sessions` package.