fix(slack): Send linking message in bot <> user DM if bot doesn't have access to channel

[iamrajjoshi]

fixes SENTRY-3BTE

the bot was attempting to send an ephemeral message in a channel that it doesn't have access to, which is why slack was sending us channel_not_found.

the solution here is if we can't send the ephemeral message, we instead send the user a dm to link their identity. because sending them a link to their message would be another slack api call, i instead settled on sending them the channel that they decided to send the link in, so they can jump to it.

[GabeVillalobos]

Should we consider implementing some sort of debounce or opt out for this? I could see this becoming obnoxious for a user who doesn't want to link their slack identity.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.

:white_check_mark: All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/integrations/slack/webhooks/event.py	33.33%	4 Missing :warning:

:x: Your patch status has failed because the patch coverage (66.66%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #77030   +/-   ##
=======================================
  Coverage   78.22%   78.23%           
=======================================
  Files        6900     6902    +2     
  Lines      306762   306825   +63     
  Branches    50283    50290    +7     
=======================================
+ Hits       239975   240037   +62     
  Misses      60428    60428           
- Partials     6359     6360    +1     

[getsantry[bot]]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you add the label WIP, I will leave it alone unless WIP is removed ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀