Remove ContextSource from ResultPrinter, refs 842

[mwjames]

This PR is made in reference to: #2748, cb6c6ad

This PR addresses or contains:

• Removes the use of ContextSource (RequestContext) from the ResultPrinter instance
• There is no particular need for the project to carry around a kitchen sink object like the ContextSource which has (and only would) increase dependency on a very specific MediaWiki concept.
• For now, ResultPrinter provides ResultPrinter::msg as interim's method in case some callers relied upon this method to fetch Message objects

This PR includes:

• Tests (unit/integration)
• CI build passed

Fixes: #842