Vue library included in main JS bundle

Merge request !12232 (merged) introduced a regression by including Vue within main.bundle.js, which has the effect of adding back the annoying dev environment message into our RSpec tests:

You are running Vue in development mode.
Make sure to turn on production mode when deploying for production.
See more tips at https://vuejs.org/guide/deployment.html

Specifically the changes made to merge_request_tabs.js include Vue components. Since merge_request_tabs.js is included by main.js and is not a child of common_vue, the Vue library moved from common_vue.bundle.js into main.bundle.js. We have previously worked around this problem by attaching a method to instantiate the Vue component to window and invoking it through that, but a better long-term solution would be to split merge_request_tabs.js from the main.js bundle so it can be a child of common_vue.

Furthermore, this MR appears to coincide with the transient job failures we're seeing in CE master (#34006). I can't yet figure out why they are related, but I'm reasonably confident that reverting !12232 (merged) temporarily will fix master.

/cc @filipa @jschatz1