accounts: add "record count", start searchable user cache

RecordCounter class created from that in activityList, and put in
accountsList. PageCount-type route standardized and made for /users
(/users/count). Created userCache, which regularly generates the
respUser list returned by /users. Added a currently dumb POST /users for
searching/pagination, GET /users is now just for getting -all- users.
go-getted expr, an expression language that seems like it'll be useful
for evaluating local searches. We don't store this data in the badger
    DB, so we can't use the nice query form provided by badgerhold.

main.go

@@ -134,6 +134,7 @@ type appContext struct {
 	pwrCaptchas                 map[string]Captcha
 	ConfirmationKeys            map[string]map[string]ConfirmationKey // Map of invite code to jwt to request
 	confirmationKeysLock        sync.Mutex
+	userCache                   UserCache
 }
 
 func generateSecret(length int) (string, error) {