Auto generate sections for all eras and their factions

This fills Feature Request #22107.
port faction description text from the wiki to the in-game help

This commit ports the text I found as the introductions to wiki
pages like this http://wiki.wesnoth.org/Drakes into the in-game
help, preserving the links to race descriptions and eras.
add portraits to decorate mainline faction descriptions
silence a warning coming from the help log channel
add race and alignment listings to the help topics for factions
Merge branch 'eras_sections'