- Namespace all admin UI URLs behind `/admin/*`.
This breaks the current admin UI URLs.
- Make Vue output build assets to `frontend/dist/*` instead of
`frontend/dist/frontend`.
- Namespace Vue static assets to `/admin/static/*`.
This commit reduces the cofusing and convoluted Vue+WebPack build URI
and static path schemes. In addition, it removes ambiguity in URLs
where non-UI URLs like `/public`, `/api`, `/webhooks` etc. were in the
same name space as UI URLs like `/campaigns`, `/lists` etc. Now all UI
URLs are behind `/admin/`, also simplifying security rules for proxies.