minor create forecast form issues

[wholmgren]

1. In the Interval label help text, we need to remove the _ from interval_label.

2. Interval value type doesn't have a help text option

3. Maybe this first needs to be resolved in core, but I'm not sure what to specify for interval value type when interval label is "event". I don't think it matters, but it's confusing.

4. Again maybe first a core issue, but should we enforce interval value type = instant when interval label = instant?