Reana-db

- Adds new disk usage retrieval methods using canonical (bytes) and human-readable (KiB) units. (`User`, `Workflow`)
- Adds Quota models which calculates CPU and disk usage.
- Adds `InteractiveSession` model.
- Adds new properties `started_at` and `finished_at` to the `Job` model, updated on status change.
- Adds `get_priority` workflow method, that combines both complexity and concurrency, to pass to the scheduler.
- Adds a possibility to configure database connection pool parameters via environment variables.
- Adds new `pending` state to `RunStatus` table.
- Adds workflow complexity property in `Workflow` table.
- Adds environment variable to configure which quotas to update.
- Changes `WorkflowStatus` table to `RunStatus`.
- Changes disk quota calculation functions to allow passing raw bytes to increase the used quota.
- Changes to PostgreSQL 12.8.
- Removes support for Python 2.

- Fixes REANA installation by pinning SQLAlchemy version less than 1.4.0 due to <https://github.com/kvesteri/sqlalchemy-utils/issues/505>.

- Adds utility to status enums to decide whether to clean workflows and jobs depending on their status.

- Adds support for Python 3.9.
- Fixes minor code warnings.
- Changes CI system to include Python flake8 checker.

- Adds initial central workflow status transition logic handler.
- Adds new audit table and logic to register actions. (`AuditLog`, `AuditLogAction`)
- Adds fixtures for better testing of database models.
- Changes user token storage to move tokens from `User` table to `UserToken` table and to encrypt them.
- Changes `Workflow` table to add a new `workspace_path` column.
- Changes default database service to use centrally configured one from REANA-Commons. (`REANA_INFRASTRUCTURE_COMPONENTS_HOSTNAMES`)
- Changes code formatting to respect `black` coding style.
- Changes documentation to single-page layout.

- Adds new method which returns full workflow name.
- Adds more granular DB configuration.
- Adds Git repository information to the workflow model.
(`Workflow.git_repo`, `Workflow.git_provider`)
- Adds user name information to the user model.
(`User.full_name`, `User.username`)
- Removes restart count information from the job model.
(`Job.restart_count`, `Job.max_restart_count`)
- Adds support for Python 3.8.