Increase DB connection pool size to 10

This is to address some errors we saw yesterday such as:

`sqlalchemy.exc.TimeoutError: QueuePool limit of size 5 overflow 10
reached, connection timed out, timeout 30`

Related flask-sqlalchemy docs:
http://flask-sqlalchemy.pocoo.org/2.3/config/#configuration-keys

app/config.py

@@ -127,6 +127,7 @@ class Config(object):
     SQLALCHEMY_COMMIT_ON_TEARDOWN = False
     SQLALCHEMY_RECORD_QUERIES = True
     SQLALCHEMY_TRACK_MODIFICATIONS = True
+    SQLALCHEMY_POOL_SIZE = os.environ.get('SQLALCHEMY_POOL_SIZE', 5)
     SQLALCHEMY_POOL_TIMEOUT = 30
     SQLALCHEMY_POOL_RECYCLE = 300
     PAGE_SIZE = 50