Assign alerts sidebar base severity

What does this MR do?

This is merge 2/3 for: #218404 (closed). Please see merge 1 for context here: !32642 (merged)

See #218404 (comment 345803729) for progression plan and caveeats.

These three merges are designed to provide the base of the rest of the work for: &3349 (closed)

In this merge we do a four things:

1. Add the severity attribute sidebar base to Alert Details
2. Disable severity dropdown edit access until the backend is complete.
3. Pre-define a grapghql mutation to modify alert severity. This might be subject to change but I think the suggested model is a reasonable guess that can be updated without much effort later.
4. Take a chance to reduce ~"technical debt" by breaking the alert severity icon into a SFC

We want to provide the most boring possible solution, clearing a clear path to progression in an iteration based approach. This is done by doing to main things:

Notes && Known Issues

• Icon used for collapsed sidebar alert status => severity-critical. This is a stop gap as I assume we will need to change this? => Follow up MR
• Severity Icon section needs to be enabled after backend is complete => Follow up MR
• Page layout needs a extra class applied to correct padding at right side => follow up MR
• Text for sidebar labels needs to be $gray-700 => Follow up MR

Proposed GraphQL mutation:

mutation ($projectPath: ID!, $status: AlertManagementSeverity!, $iid: String!) {
  updateAlertSeverity(input: { iid: $iid, severity: $severity, projectPath: $projectPath }) {
    errors
    alert {
      iid,
      severity,
    }
  }
}

Screenshots

Current State - Edit Disabled

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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