content: Refactor normalization of line endings code

content: Refactor normalization of line endings code

The code that normalizes line endings ("\r\n" and "\r" to "\n") and
trims trailing whitespace is buried in Parser::preSaveTransform(), and
was duplicated to TextContent in 96b6afb31dfcff, as non-wikitext content
models should still be normalizing line endings.

This splits the duplicated code into
TextContent::normalizeLineEndings(), and utilize it in the Parser.
Additionally, expand the documentation of
TextContent::preSaveTransform() to document that subclasses should make
sure they normalize line endings during the PST stage.

And remove a useless rtrim() call from WikitextContent that did nothing.

Change-Id: I9094c671d4bbd23d75436f8f1d682d6dd6e6d2fc