WDK: results table: column help: enable html

[aurreco-uga]

(1) make attribute help an element in the rng file, ( for model backward compatibility just name it 'help' and client will choose element over attribute if exists)

(2) update the tooltip in the client table (dave said mesa knows how to do it)

example of column that will benefit from this:
https://plasmodb.org/plasmo/app/workspace/strategies/import/fa20c76032cb87cc
(any column where we show terms that need definition)