Adding GetNewMessagesAlert hook and wgUserNewMsgRevisionId JS global

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