fix: load slack channels earlier

[eschutho]

SUMMARY

Loading slack channels earlier to ensure that the method dropdowns have the correct value when they are loaded. Instead of loading slack channels when the slack option is selected, I am now loading the options when the notification method is rendered. That way, we know if we need to use slack v1 or v2 by the time the notification method options are rendered (since we filter out one of the options, so that only one slack option displays)

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

no visual changes

TESTING INSTRUCTIONS

If the feature flag is on, but the scopes are missing, a v1 (textarea input field) should display for slack.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[sadpandajoe]

@supersetbot label 4.1

[betodealmeida]

This line shouldn't be necessary, right?

[eschutho]

It's to help typescript assert that the value exists, rather than runtime execution. e.g., for NotificationMethodOption as it doesn't include undefined. I could coerce the type, but I think that sometimes looks like a shortcut, too. wdyt?

[eschutho]

I'm going to merge, but if you suggest another way to approach this, I'll put it up in a new PR.