vector: Apply content text style via .mw-body-content instead of #bodyContent

Font size, line height and positioning styles related to content
rendering should be re-usable for applying in other contexts.
Not just the #bodyContent div.

The use of classes for content styling has been recommended and generally
agreed upon, specifically in the context of the RFC about scoping site CSS:
    https://www.mediawiki.org/wiki/Requests_for_comment/Scoping_site_CSS

This change takes the first step toward introducing a reusable class
throughout the UI to apply content-specific styles where needed.

This also makes it possible to resolve a bug in which VisualEditor
does not receive the correct styles because it creates the edit surface
outside the #bodyContent (but within #content) and had no way to
receive the same styling.

Bug: 63985
Change-Id: I66c56c577bad064261c3e4a62a1c1b2566d53972
(cherry picked from commit 9c8592fe786b399d0a321d65d681ba93abd7ce42)