Reana-server

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- Upgrades REANA-Commons package using latest Kubernetes Python client version.
- Pins Flask and Invenio dependencies to fix REANA 0.6 installation troubles.

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- Fixes bug with big file uploads by using data streaming.
- Adds user login endpoints using OAuth, currently configured to work with CERN
SSO but extensible to use other OAuth providers such as GitHub, more in `Invenio-OAuthClient <https://invenio-oauthclient.readthedocs.io/en/latest/>`_.
- Adds endpoints to integrate with GitLab (for retrieving user projects and creating/deleting webhooks).
- Adds new endpoint ``/me`` to retrieve user information.
- Improves security by allowing requests only with ``REANA_URL`` in the host header, avoiding host header injection attacks.
- Initialisation logs moved from ``stdout`` to ``/var/log/reana-server-init-output.log``.

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- Adds new endpoint to compare two workflows. The output is a ``git`` like
diff which can be configured to show differences at metadata level,
workspace level or both.
- Adds new endpoint to retrieve workflow parameters.
- Adds new endpoint to query the disk usage of a given workspace.
- Adds new endpoints to delete and move files whithin the workspace.
- Adds new endpoints to open and close interactive sessions inside the
workspace.
- Workflow start does not send start requests to REANA Workflow Controller
straight away, instead it will decide whether REANA can execute it or queue
it depending on a set of conditions, currently it depends on the number of
running jobs in the cluster.
- Adds new administrator command to export and import all REANA users.

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

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

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