Adding GetNewMessagesAlert hook and wgUserNewMsgRevisionId JS global

This hook allows extensions to disable or modify the new messages
alert ('orange bar of doom') while still allowing the user_newtalk
table to be updated.

The wgUserNewMsgRevisionId JS global allows gadgets and extensions
to create their own new message alerts on the client side.

I also threw in a few comment updates for good measure!

See also Echo change I3f35a56b which utilizes this.

Bug: 47962
Change-Id: I2105bdd2bcd5b27f6f36ec8d8fa7fa99d60a2d82