Volunteering page should allow filtering to union of (my shifts) and (shifts with spaces)

[waiwaing]

The volunteering page doesn't currently let you filter to see both shifts you've signed up for as well as shifts that are still seeking volunteers. This is arguably the most useful view for volunteers once people are on site.

The relevant filters are currently:

• Only show my shifts
• Hide full shifts
• Only show understaffed shifts - skimming the code, this is not the opposite of full because there
  is a minimum staffing level above which a shift can still have volunteers but is not understaffed

I'm not sure what the right UX approach is here, but potentially projecting the internal concern about understaffing vs staffed but not full is confusing.

Possibly a change to a four-way state:

• Show all shifts
• Show my shifts + understaffed shifts
• Show my shifts
• Show understaffed shifts

Happy to submit a PR if this approach looks sensible (or something else is proposed)