Fixed major bug EXT-6661 (chat log paths reset back to default)

 * Removed unused InstantMessageLogFolder
 * Restored initializing logic of InstantMessageLogPath value from 1.23 to share setting between viewers.
 * Restored unused in 2.0 "LogChatTimestamp" and "LogChatIM" settings to avoid removing of them in 2.0
 * Restored default value of the "Save local chat log" setting from 1.23 & replaced with 2.0 own setting (LogNearbyChat was: LogChat)

Reviewed by Igor Borovkov & Kent Quirk at https://codereview.productengine.com/secondlife/r/193/
(transplanted from 43297b92c1bb7487dab6610bea61acac4c355878)