Reana-commons

- Adds `get_disk_usage` utility function to calculate disk usage for a directory.
- Adds `Yadage` workflow specification loading utilities.
- Adds workspace validation utilities.
- Adds `Snakemake` workflow engine integration.
- Adds custom objects API instance to k8s client.
- Adds available worklow engines configuration.
- Adds environment variable to define time between job controller connection checks.
- Adds cluster health status endpoint.
- Adds OpenAPI specifications with respect to user quotas.
- Changes `workflow-submission` queue as a priority queue and allows to set the priority number on workflow submission.
- Changes OpenAPI specifications with respect to turning `workspaces` endpoint into `info`.
- Changes publisher logging level on error callback.
- Removes support for Python 2.

- Adds support for glob patterns when listing workflow files.
- Adds support for specifying `kubernetes_memory_limit` for Kubernetes compute backend jobs.

- Adds new functions to serialise/deserialise job commands between REANA components.
- Changes `reana_ready` function location to REANA-Server.

- Adds new configuration variable to toggle runtime user jobs clean up depending on their statuses. (`REANA_RUNTIME_KUBERNETES_KEEP_ALIVE_JOBS_WITH_STATUSES`)
- Adds central class to instantiate workflow engines with more resilience. (`workflow_engine.create_workflow_engine_command`)

- Adds support for Python 3.9.
- Fixes minor code warnings.
- Fixes a helper function that calculates directory hashes.
- Changes OpenAPI specifications with respect to sign-up form.
- Changes OpenAPI specifications with respect to email confirmation.
- Changes CI system to include Python flake8 checker.

- Adds support for restarting yadage workflows (through `accept_metadir` operational option).
- Allows `htcondor_max_runtime` and `htcondor_accounting_group` to be specified for HTC jobs.
- Adds new field in REANA-Server OpenAPI spec to return server version.
- Changes CI system from Travis to GitHub Actions.