Took the NotesApp out of the global scope

What does this MR do?

Instead of exposing the notes app (the discussions in Issues and Epics) instantly when the module is imported, we allow the modules, requiring the notes app decide on their own when to initialize the notes app.

This is the very first step in optimising the loading performance of the Knowldge_Design_Single route.

This MR allows making sure that the notes/comments do not get processed outside of initShowIssue() that is going to be explicitly controlled and called on the next step of performance optimization for the Single Design route (next MR).

Screenshots or Screencasts (strongly suggested)

No visual changes

How to setup and validate locally (strongly suggested)

1. Visit any issue or epic with discussions
2. Make sure the discussions still get loaded and rendered

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Related to gitlab-com/www-gitlab-com#11918 (closed)