Allow user to specify how far in the past people connecting to the chat will see.
This is important on Learn@WU because people joining the chat room later want to
see the full conversation and before this was not settable from the UI. (Now defaults
to 4 hours) Useful also when somebody accidentally refreshes the chat after the
timewindow has passed.
Took this chance to ease a bit every future development: removed stored procedures
slowing greatly any change on the data model in favor of plain sql statements.
Incremented usage of cache.
Removed duplicated database operation when managing transcripts.
Fixed transcript name not being shown when deleting.
