remove kwargs from default channel implementation send methods

[erohmensing]

Proposed changes:

• remove kwargs from default channel implementation. If users want to support these kwargs and e.g. passing them along to another function (from elements to text with buttons, for example), they can still implement their own send_elements function that does just that.
• fixes the following multiple values error we have seen with migrating from a send_elements workaround before send_custom_json existed:

  File "c:\anaconda3\envs\rasa_latest\lib\site-packages\rasa\core\channels\chan
nel.py", line 255, in send_elements
    recipient_id, element_msg, element.get("buttons", []), **kwargs
TypeError: send_text_with_buttons() got multiple values for argument 'text'

Status (please check what you already did):

• made PR ready for code review
• added some tests for the functionality
• updated the documentation
• updated the changelog
• reformat files using black (please check Readme for instructions)

[akelad]

do we even need kwargs in the function definitions then though?

[erohmensing]

i believe so, because we still want them supported in the subclassed channels e.g. here https://github.com/RasaHQ/rasa/blob/master/rasa/core/channels/telegram.py#L42, if we remove them from the base class then it will yell at unexpected arguments (i think, but maybe that's something idk about python)

[akelad]

ok you can merge

[erohmensing]

cool, no rush, i thought i'd sanity check with fede or someone before merging... if i ever get out of meetings lol