Make SQLiteModelStore suitable for testing.

Made the CreateTables method virtual in the SQLiteModelStore, such that
tests that require their own modes could override that method easily.
Add RelatedModelsCollection.

RelatedModelsCollection is a class for many-to-many collections that the
models can have.
Add time entry tags support.
Add WorkspaceUserModel.
Add ProjectUserModel.
Merge branch 'roosmaa-m2m'

Implment the many-to-many relationships foundation as well as
relationships between time entries and tags, workspaces and users
and projects and users.

Closes #1