Implement talk page messages

[L235]

Feature request: add an option to leave both boilerplate messages and custom messages for all or selected socks/suspected socks. The messages can include:

• {{subst:Socksuspectnotice}}, when the filing editor did not notify the socks and their input might be useful
• A stronger message to the effect that the suspected sock should present any response expeditiously and should not edit further before responding to the SPI (e.g. likely meatpuppets who should be given a chance to admit it)
• {{subst:Uw-sockwarn}}, when blocking the other socks but not blocking the master (e.g. unintentional violations)
• Custom messages

The easiest way to implement this, I imagine, is just to add another checkbox to the "block/tag socks" menu, since all the logic for getting users etc. is already there.

[GeneralNotability]

I think this might be better as a separate view from the block view, but I'm not 100% set on that - the block view is just getting kind of crowded and this will be another drop-down.