Reana-server

- Changes rate limiting defaults to allow up to 20 connections per second.
- Fixes minor code warnings.

- Fixes REANA \<-> GitLab synchronisation for projects having additional external webhooks.
- Fixes restarting of Yadage and CWL workflows.
- Fixes conflicting `kombu` installation requirements by requiring Celery version 4.
- Changes `/api/you` endpoint to include REANA server version information.

- Adds new endpoint to request user tokens.
- Adds email notifications on relevant events such as user token granted/revoked.
- Adds new templating system for notification email bodies.
- Adds possibility to query logs for a single workflow step.
- Adds endpoint to retrieve the workflow specification used for the workflow run.
- Adds preview flag to download file endpoint.
- Adds validation of submitted operational options before starting a workflow.
- Adds possibility to upload empty files.
- Adds new block size option to specify the type of units to use for disk size.
- Adds a possibility to upload new workflow definitions before restarting a workflow.
- Adds new command to generate status report for the REANA administrators; useful as a cronjob.
- Adds user token management commands to grant and revoke user tokens.
- Adds support for local user management.
- Adds pinning of all Python dependencies allowing to easily rebuild component images at later times.
- Fixes bug related to rescheduling deleted workflows.
- Changes `REANA_URL` configuration variable to more precise `REANA_HOSTNAME`.
- Changes workflow list endpoint response payload to include workflow progress information.
- Changes import/export commands with respect to new user model fields.
- Changes submodule installation in editable mode for live code updates for developers.
- Changes pre-requisites to Invenio-Accounts 1.3.0 to support REST API.
- Changes `/api/me` to `/api/you` endpoint due to conflict with Invenio-Accounts.
- Changes base image to use Python 3.8.
- Changes code formatting to respect `black` coding style.
- Changes documentation to single-page layout.

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

- 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`.

- 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.