Volunteering / Commmunity Event function improvements

[edwh]

• Add hold/release function.
• If you're logged out and click to add a volunteer op then you get the forced login modal, which you can't get out of. Ditto community events.
• For volunteer ops, we used to have an "apply by" date. Add it back.
• If you land on the page for volunteer ops/events for a group, which can happen if you're sent the link, and try to add one, then you need to be signed up to the group. Or perhaps more generally we should provide a way for people to select and join a group that they're not already a member of.
• If you add a volunteer op with a date, then immediately add another, the dates are preserved from the old one, though the rest of the form is empty.