ref(messaging): Extract ABC for linking and unlinking teams

[RyanSkonnord]

Rough sketch of how we would generify Slack's team-linking and -unlinking views for other messaging integrations. A few Slack-specific details remain (around the HTML templates and such) but these are the broad strokes.

A working use case for MSTeams and Discord would require some details on how to generify interactions with Slack channels.

Note that this PR only covers the execution of the linking or unlinking operation and the view that is rendered when it is done. The in-client command and the webhook that the client calls are out of scope.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 73.23232% with 53 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/messaging.py	71.17%	37 Missing and 12 partials :warning:
...c/sentry/integrations/slack/views/link_identity.py	0.00%	1 Missing and 1 partial :warning:
src/sentry/integrations/slack/views/link_team.py	89.47%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #76214       +/-   ##
===========================================
- Coverage   88.38%   78.16%   -10.22%     
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  Files        2977     6902     +3925     
  Lines      185278   306763   +121485     
  Branches    30514    50272    +19758     
===========================================
+ Hits       163764   239794    +76030     
- Misses      15521    60556    +45035     
- Partials     5993     6413      +420     

[sentaur-athena]

The structure and abstractions look good! Please ping again when it's ready to review.