feat(slack): add Slack event handling for team joins and messages

[JisanAR03]

This pull request introduces a new feature to handle Slack events in the blt project. The most significant changes include adding a new view for Slack event handling, integrating it into the URL routing, and implementing various functions to process Slack events such as team joins and messages.

Slack Integration:

• blt/urls.py: Added import for slack_events and integrated a new URL path to handle Slack events. [1] [2]
• website/views/slack_handlers.py: Created a new file with the slack_events view and supporting functions to handle Slack events, verify Slack signatures, and process different types of Slack messages.

need to add in .evn:

• SLACK_TOKEN and SIGNING_SECRET deatails

@DonnieBLT sir , can you please review this pr?
fix #2972

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ AttributeError: 'NoneType' object has no attribute 'encode' /slack/events View Issue

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