Livebridge

* .readthedocs.yml added
* Fix for unclosed S3/SQS client-sessions

This release adds possibility to inactivate bridges by setting control property to **active: false**. All other cases (**active: true** or property missing) will see the bridge as active.

* Support for control-data property **active** bridges added.
* Upgrade dependencies
* Add requirements-dev.txt for cleaner dependencies
* Partial fix for unclosed DynamoDB client session.
* Web-UI: hide and show inactive bridges in listing.
* Web-UI: Open target -from after adding a bridge.
* Web-UI: copy existing source to add-form.
* Web-UI: Add animated loader.
* Web-UI fix: reset add-target form after cancel.
* Remove inactive bridges from control-data.
* Add bridge hash to __repr__ string.
* Add another test for control-data changes.
* Improve tests.

Known issue:
* unclosed session for DynamoDB client. Not critical for Livebridge, will be fixed upstream.

* Add abstract method stop() to polling sources.
* Add handling for boolean config vars in Web-UI.

* New web-frontend for editing control-data, see [docs](https://livebridge.readthedocs.io/en/stable/control.htmlcontrol-bridges-via-web-ui) for more information.
* Ability to add auth credentials via ENV variables by using **env.LB_** notation, [docs](https://livebridge.readthedocs.io/en/latest/control.htmlenv-var-notation-for-service-credentials).
* Improved web-API.
* Upgraded Python dependencies

* Improve sequential processing of new posts of a source.
* Improve retry mechanisms for failed target distribution, introducing **LB_RETRY_MULTIPLIER** and **LB_MAX_RETRIES** for configuring retry delay.
* default behaviour of watching for local control-data file changes deactivated. Use **LB_CONTROLFILE_WATCH=true** to force this checks.
* Improved documentation

* Add and remove only changed bridges after control data changes.
* Style consistency with flake8