Fix content validation for existing wiki pages

What does this MR do?

Spotted while doing QA on !36729 (merged):

This new validation didn't work correctly yet because of two reasons:

• When loading the wiki page in the #update controller action, the content in attributes[:content] won't be initialized which causes content_changed? to return true. Our factory on the other hand sets attributes[:content] as part of its initialization.

  We can fix this by using raw_content instead in content_changed?, and verify this in the specs by reloading the existing page from the wiki.

• Gollum converts CRLF line endings ("\r\n") to LFs ("\n"), so we also need to do this conversion in WikiPage#content_changed?.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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