Add availability to issue template

[ShMcK]

Should be easier to schedule if we can know speakers availability.

[apike]

Seems like a good idea, though the wording of this is confusing I think – if the idea is to try and only do VanJS on Thursdays from now on, we could ask:

Which dates, particularly Thursdays, are you likely available? (e.g. "Most Thursdays with 4 weeks notice", "June or later", etc.)

Or since we don't seem to have locked down a cadence yet, for now we could ask more generally:

What month(s) would you be available?

[ShMcK]

Great ideas, would be great if we could eventually work towards a regular "third Thursday of the month".