Reana-server

- Improves REST API documentation rendering.
- Enhances test suite and increases code coverage.
- Changes license to MIT.

- Harmonises date and time outputs amongst various REST API endpoints.
- Pins REANA-Commons, REANA-DB and Bravado dependencies.

- Adds support of Serial workflows.
- Adds API protection with API tokens.

- Adds support of Common Workflow Language workflows.
- Adds support of specifying workflow names in REST API requests.
- Improves error messages and information.

- Initial public release.