Maximum user limit

This adds a configurable maximum user limit. To enable it, set `maximumUsers` in the default or a provided config file.

This also adds a `GET /accountBuffer` route which, if `maximumUsers` is set, will return the number of accounts left until the limit is hit. It does not divulge the user count.

This still needs tests which I will add tomorrow and manual testing in the example apps.

Closes #286
Any query planner worth a damn should handle them both the same way. Postgres does. From a table with ~700K rows:

```
db=# explain select count(*) from table;
                             QUERY PLAN                              
---------------------------------------------------------------------
 Aggregate  (cost=61555.45..61555.46 rows=1 width=0)
   ->  Seq Scan on table  (cost=0.00..59807.76 rows=699076 width=0)
(2 rows)

db=# explain select count(id) from table;
                             QUERY PLAN                              
---------------------------------------------------------------------
 Aggregate  (cost=61555.45..61555.46 rows=1 width=4)
   ->  Seq Scan on table  (cost=0.00..59807.76 rows=699076 width=4)
(2 rows)
```

And you can't index count(), it's not a row function.