Make chair counts easier to understand

[saadabdali]

@beaudrykock-moi - I spent some time exploring the counts.

On the one hand: this is a small change that I think is ultimately more logical than what we have today.

On the other hand: I also feel I actually ought to ask more questions about who the audience for this change is, and what context they're operating in. And that there's a lot more we could provide here if we knew more about that.

What's in this PR

• In the Facility card (for CUSTODY and CARE users), we replace the In transit count with a held count.
• So now the card shows three numbers: Available, Held, and Occupied. These three numbers are always guaranteed to add up to your online capacity (I'm thinking of online capacity as total capacity, less any chairs marked unavailableOccupied or unavailableUnoccupied by an admin).
• Because 'in transit' is still useful, we show that number on a badge directly on the Transit tab.

Here's what that looks like for CUSTODY:

CARE will also see the updated Facility card, but they don't have a Transit tab, so they don't get that badge.

Why we might merge this

The mental model here is clearer, I think:

• the numbers in the Facility Card are always chair-centric - they're trying to give you an overall picture of how the capacity is currently allocated. In fact, if you look at the old facility card with fresh eyes, it's actually a little strange - it says 10 chairs available in huge font, and then below that, it says 2 in transit. Two chairs in transit?
• The tabbed lists below that (Transit, Custody, Released) are workflow-centric, and they cut across the natural chair categories. (Transit is just a subset of held chairs; Custody includes both held chairs and occupied chairs; and Released includes some people who are outside the chair accounting.)

Why we might want to do something else (instead, or in addition)

• In this PR, CARE users can no longer see the number of people in transit from the main page. (They can still see it in the detailed breakdown when they click Manage capacity).
• Adding a count badge on the Transit tab header uses more horizontal space, which looks fine on my phones but may exacerbate clipping on phones with enlarged fonts.
• There's a question of whether this actually helps the right people. The current Facility card is confusing, but which roles at RESET actually care about that? If it's CARE, don't they already get what they need from the detailed breakdown on the manage-capacity page? Instead of trying to cram the chair-accounting onto the page header, should we actually offer a full-page, tablet-optimized (rather than phone-optimized) experience? Instead of coarse buckets (held/occupied/available), is it helpful to show a more complete sequence diagram of all the states? In addition to counts, do we actually want to show the underlying Holds in every category? This all depends on who we're imagining the audience for this is, and what they're trying to do.

[saadabdali]

One more quick thing:

• The PR also includes a new doc to document the state machine for deflections, which I think is not succinctly described anywhere else (docs/holds-state-machine.md). And added pointers to this file from the ARCHITECTURE.md and a couple relevant places in the code.