Remove/Override .right-sidebar influence

What does this MR do?

Refactor alert details sidebar CSS

• Remove/Override .right-sidebar influence
  • on alert sidebar assignees
  • on alert sidebar status
  • on alert sidebar todo
• update tests

Changelog: added

Motivation

1. In #287755 (closed), a sidebar will be created that does much of what the alert details sidebar does, but it has slightly different CSS because I will be using GlDrawer and the sidebar does not need to be collapsed. With these changes, the components affected in this MR are being updated to conditionally render elements via flags instead of via CSS classes, which I think is the right way to conditionally render elements anyways.
2. Per a Slack conversation, .right-sidebar and all the related CSS is legacy from the first sidebar and is being removed as part of the main sidebar in &5302. From then forward, they will move to removing this CSS.
3. The Todo component was only slightly updated for a few reasons: it was too complex to untangle from .right-sidebar in this MR and, thus, warrants its own MR; I updated the border because the current borders do now use a pajamas-sanctioned color and the other two components got their border-colors updated and it looked odd to not update this one as well.

• Sidenote: The collapsed sidebar has a bug with the assignee not disappearing and that is being addressed with #327053 (closed)

How to test

https://gitlab.com/-/snippets/2046318

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #287755 (closed)