Decouple content-wrapper and content-wrapper-margin

• Requirements (issue)
• Tasks (epic)
• Full Context Spike MR (permanent draft)
• All MR Parts...
  • Decouple content-wrapper and content-wrapper-margin
    • Part 1 - Setup for responsive top nav redesign (depends on parent MR)
      • Part 2 - Top nav responsive view (depends on parent MR)

What does this MR do?

This MR pulls the margin: part of content-wrapper into it's own class, so that it can be reused by the upcoming top nav responsive redesign.

Screenshots (strongly suggested)

What should I be looking for?

• There are 3 layouts that are effects: terms, application, fullscreen
  • The Terms & Conditions page tests the terms layout
  • The Project page tests the application layout
  • The Web IDE tests the fullscreen layout
• You'll notice that we actually fix a layout issue in the Web IDE with system footers 🎉

Description	Before (on master)	After (this MR)
Terms & Conditions page (inspect)
Project
Project (with perf bar)
Project (with perf bar + system header)
Project (all + inspect)
Web IDE
Web IDE (with perf bar)
Web IDE (all + inspect)

2021-06-06 smoke test

Project with system head and foot	Web IDE

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not 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.

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.

Related to #301143 (closed)