slack: add DM contact method and use for linking

[mastercactapus]

What problem would you like to solve? Please describe:
Linking and managing the association with a Slack account is complex and "magical" currently.

• The only ways to "link" a Slack user to GoAlert is by creating a test alert or having an admin use a special-case tool (goalert-slack-email-sync)
• The status of being linked is invisible
• Makes it difficult for @ mentions to work for schedules
• Blocker for Update slack group users to match the current on-call rotation. #1228

Describe the solution you'd like:
Allow users to add a contact method for Slack and use this to identify actions coming from Slack.

Describe alternatives you've considered:
A suggested alternative was to add a /goalert link command, but this keeps the linking status invisible, and users/devs/admins are unable to remove the link (e.g., for testing).

Additional context:
Notes from the design session:

• need to keep auth_subjects in sync with Slack contact methods
• existing links must result in new, verified, contact methods
• existing flows should remain untouched for MVP

To be added:

• New contact method type SLACK_DM to be added
• Mimic phone number entry but use @ instead of +
• same format as channel notifications
• Same validation & test flow
• Use metadata column to store and display user name
• store value as team_id:user_id
• add cm_id as a nullable column to auth_subjects defaulting to null (on delete cascade)
• create auth_subjects entry when creating new contact method (with cm_id set)
• new engine compat module
• create verified contact methods for providers starting with slack: and null cm_id, update cm_id

MVP+1:

• update to use CM as the source of truth for slack user lookup
• update slack username over time (maybe when a message is sent up to once per day?)

[mastercactapus]

We will try a new strategy instead of the giant long-lived feature branch to move this along: merging functionally verifiable pieces before the feature is ready.

Current work is in branch feat/2712-slack-dm-contactmethod

Remaining work:

• remove user search GraphQL stuff from the branch
• break existing work into PRs
  • Slack caching refactor (generics)
  • Add new CM type with GraphQL API (disabled: no registered sender, API/data only)
  • compatibility manager to sync CM with auth_subjects (disabled)
• next: status update solution (e.g., update sent alert notifications)
• next: enable disabled pieces

For the disabled parts, let's either have commented code or a temporary flag, so it's easy for a reviewer/dev to enable validating PRs but disabled in production.

[mastercactapus]

Last items for the finish line:

• status updates: switch to per-contact-method behavior #2824 needs to get merged
• Validate and fix any remaining issues with slack dms (e.g., feature flag-on)
• Remove the feature flag (always on)