Single issuable vue app for Issue page

What does this MR do?

Current WIP from pairing with @sstern

Todos:

Separate MRs for:

1. Single export from issue_show that calls new Vue twice
2. Feature flag in HAML + pages/index.js
3. Add issue.sentry_errors ..etc

Todos:

• CE/EE considerations; related issues is moving to core which mostly eliminates the need