Provide users guidance in making decisions about the time components of their volunteer needs

In VOL-140 and the related brainstorming doc we've discussed:

• the various time components of a volunteer need (duration, start time, and end time)
• a few different notions of "fuzzy" needs, and
• the confusion over what a flexible need is.

Well, I haven't addressed all of it, but I'm about to make a pull request for some improved help text and a quasi-wizard-like addition to the needs UI.

The proposed change introduces a select box that first asks the user to choose a schedule type.

• If the user selects a set shift, only the start and duration will be shown, and the end is blanked out.
• For a flexible timeframe, all three fields are shown – this allows a specification such as "I need 10 hours of filing done sometime this week."
• For "Open until filled," none of the components are shown to the user. The start is set to today's date and the other fields are blanked. Example use case: I need a volunteer graphic designer.