"Placeholders" quality check ignores "`smalltalk-format`" flag in `.po` file

[ikedas]

Steps to reproduce:

1. Create a template .pot including smalltalk-format flag:

   #. (list.name,domain)
   #: default/mail_tt2/listmaster_notification.tt2:345
   #, smalltalk-format
   msgid "Listmaster: list %1@%2 at 100 percents error"
   msgstr ""
   
   ...

2. Add it into a project on Pootle, and make that project use "l20n" Quality Checks.

3. On Pootle GUI, add a translation including escaped "%" character:

   ...
   
   msgstr "Listmaster : la liste %1@%2 est à 100%% d'erreur"

Results (Expected/Actual):

Expected result

A smalltalk format string "starts with ‘%’ and is followed by ‘%’ or a nonzero digit (‘1’ to ‘9’)". [ref]

So if smalltalk-format flag exists, the "%%" in the translation should be treated as a single "%" sign and shouldn't be treated as an error.

Actual result

The "%%" is treated as a critical error:

Environment (i.e. 'pootle --version', DB, OS, Browser):

$ pootle --version
Pootle 2.8.2 (Django 1.10.8, Translate Toolkit 2.2.5)