auth: link slack user

[tony-tvu]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
This PR facilitates linking slack users to their GoAlert accounts, which would allow them to acknowledge or close an alert from the Slack UI once linked.

Out of Scope:
Devtool's mock slack server has been updated to account for block elements

Screenshots:

1. Alert sent to unlinked slack user

2. Clicking 'Link Account' button routes them to GoAlert's profile page to confirm linking

3. Once linking is confirmed, it'll automatically acknowledge or close the alert based on the selection at stage 1

4. Linked user can now Close the alert from the slack UI

[mastercactapus]

Here's what I think we have left:

• Update the messaging to be a little more informative, like: Slack user <User.Name> (@<User.Username>) from <Team.Domain>.slack.com
• We should show errors in the dialog before closing (e.g., network issue)
• Unknown/invalid/expired tokens should have consistent messaging
• We should probably add the details in the token, or at least a signature -- it's too easy to make a URL that will link a user in a way they didn't expect; we can have a query to validate the token before the user confirms

[mastercactapus]

I moved the metadata to a table column, that way, to get any info, you need a valid GoAlert session AND the token/url, and also handles ensuring the messaging the user gets is also validated and isn't subject to a malicious/misleading link.

[mastercactapus]

Leaving integration test out-of-scope; we'll add that as a test when the mockslack server gains a UI