Backend: Unfurl with views using Slack Block Kit UI

This issue is to build on #358677 to create the link unfurl views in #337481 using views made with Slack Block Kit.

An example of the Slack Block Kit code to make a merge request view is in this comment #337481[Unfurling_states_-_issues.png] (comment 730095948).

We should implement Slack Block Kit views for:

• Issues
• Merge Requests

(this is being clarified in #337481 (comment 907440946))

Questions

Should the views be implemented using templates, or a DSL, or a gem (like this)?

See also

• Design UX issue #337481

Availability & Testing

Suggestion:

• add higher level unit or request specs that assert the correct response content is being sent to the slack hook for requests from slack. (issue and merge requests)
• Future state: write e2e tests against Slack app.

Considerations that may already be considered:

• Edge cases and empty states - Example: (20 assignees on an issue, or no assignees, long issue or MR titles)
• Consistency between Slack app / Slash commands
• For slash commands, consider how a self-hosted instance with relative urls will generate links.