Move alert_details components to vue_shared

What does this MR do?

Move alert_details components to vue_shared

• abstract reused severity_levels constant to a global level
• move graphql files to graphql_shared
• move tests to vue_shared

Screenshots (strongly suggested)

Page	Screenshot
The page!
The tracking! I navigated to the list view, changed the status of the alert, navigated to the details view, and changed the status of the alert, so there is a page view event, structured event, page view event, and structured event

Actions tested

• load default tab
• load metric tab (I did not have any local data for this, but I verified the graphql request succeeded)
• load activity feed tab
• assign alert
• change alert assignee
• add todo
• toggle sidebar
• create incident from alert

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

Related to #296717 (closed)