Update Jira subscriptions list to use Vue

What does this MR do?

Related to #321441 (closed)

• Move subscriptions list to be rendered in Vue.
• Extract some shared template to group_item_name.vue.
• Refactor all calls to AP object to have fallback when not in the context of an iframe. Also consolidate into a single util file and update all other imports.
• Update existing specs and add specs to cover all code (new and old).

Screenshots (strongly suggested)

Scenario	Before	After
0 linked namespaces
1 linked namespace
2 linked namespaces (with success message)

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
• Code review guidelines
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• 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