Add separate config item for failed login count

It’s confusing that changing `MAX_VERIFY_CODE_COUNT` also limits the
number of failed login attempts that a user of text messages 2FA can
make.

This makes the parameters independent, and adds a test to make sure any
future changes which affect the limit of failed login attempts are
covered.

app/config.py

@@ -148,6 +148,7 @@ class Config(object):
     TEST_MESSAGE_FILENAME = 'Test message'
     ONE_OFF_MESSAGE_FILENAME = 'Report'
     MAX_VERIFY_CODE_COUNT = 5
+    MAX_FAILED_LOGIN_COUNT = 10
 
     # be careful increasing this size without being sure that we won't see slowness in pysftp
     MAX_LETTER_PDF_ZIP_FILESIZE = 40 * 1024 * 1024  # 40mb