feat: Show how many cards a list holds

A board level switch, off by default. With it on, each list puts its card
count on the add-card button. Useful to a board that is being kept to a size
and noise to one that is not, which is why it is a choice rather than always
on.

The count is every card in the list, not the ones a filter leaves visible.
Those answer different questions: how full is this list, versus how many match
what I am looking at right now. The first is the one a counter is for, and it
is also what the same switch counts in Pro, so a board keeps its meaning
across an upgrade.

The column matches Pro's name, type and default, so the setting survives that
upgrade rather than being dropped on the way.

client/src/models/Board.js

@@ -36,6 +36,7 @@ export default class extends BaseModel {
     defaultCardType: attr(),
     limitCardTypesToDefaultOne: attr(),
     alwaysDisplayCardCreator: attr(),
+    showCardCounter: attr(),
     displayCardAges: attr(),
     expandTaskListsByDefault: attr(),
     context: attr(),