Use Slack Block Kit client library

[alysivji]

I've used the Builder pattern to simplify the process of creating UI blocks, but there are libraries that are maintained that do the same thing. New components would be available without having to add them myself.

Need to investigate which library is best out of:

• https://pypi.org/project/slackblocks/
• https://pypi.org/project/blockkit/
• https://pypi.org/project/slacktools-blockkit/ (there are lot of other tools by the same creator)

Then migrate our existing blocks.