add flag for turning off the "Become a..." sponsor messages

[bridgetkromhout]

The "Become a..." choices are toggled off on past events. We should add that as a toggle-able option for current events, as sometimes sponsorships sell out.

[mattstratton]

I think this is a dupe of #93

[bridgetkromhout]

I think this is a dupe of #93

Correct. Closing this dup.