use resource loader for message delivery where possible (bug 43409)

make mw.mobileFrontend.msg first try mwMobileFrontendConfig and if
message not there use  mw.message where available
change addMessages to read directly from a list of known modules to keep support
for stable which has no RL

remove unused messages from i18n

Change-Id: I14b60ac183b2a995e2400c2cef29f63dcc3db37f
Merge "use resource loader for message delivery where possible (bug 43409)"