Separate Diesel support into a separate crate

[lawliet89]

• Separate the functionality into a separate crate (Separate Diesel support into separate crate #69)
• Use r2d2-diesel (Use connection pools with r2d2-diesel #70)
• Generic DB agnostic implementation (Genericise r2d2 implementation #71)
• Implement migration for Library (Implement migration for MySQL #73)
• Implement Migration in CLI (Add migration operation to the CLI #77)
• Improve test harness wrt database servers setup (maybe with Docker Compose to run the databases) (also perhaps via integration tests instead)
• Support SQLite (Add SQLite support #78)
• Support Postgres (Implement PostgresSQL support #79)
• Add tests for "non default" DB schema
• Add CLI tools for managing users
• Improve documentation

[adeadman]

Looks good. One thing I felt was missing from the MySQL implementation was a way of adding new users to the database, the way the csv backing can. Let me know if I can help with any of the tasks.