Move more functions to the pdfLetter class

[eileenmcnaughton]

Overview

Move more functions to the pdfLetter function

Before

Functions on 'common' class that isn't really common and is only used by this class within git universe

After

More functions moved back to the 'real' class. Instances of 'self' replaces to make the process less fraught

Technical Details

This 'common' form isn't really common - it's silly. This PR removes all
the gotcha instances of 'self::' to make it easier to decommission this file

Note that there are no calls to the class outside core in git universe

Unit tests cover the code in question & any fail would be a hard fail

Comments

[civibot]

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[eileenmcnaughton]

@colemanw this next one is passing now - it would hard fail on tests if not right

[colemanw]

Looks good