tools: Add new script to sync translations.

We just learned we should be using the "onlytranslated" mode of
Transifex.  Since the command is getting a bit complex (and you need
to remember to run `makemessages` first), it makes sense to have a
tool for it.

docs/translating.md

@@ -103,8 +103,8 @@ translating, you should check out the
 4. Translators translate the strings in Transifex.
 
 5. The translations are downloaded back into the codebase by a
-   maintainer, using `tx pull -a -f --mode=developer` (since we don't
-   use the Transifex review flow).
+   maintainer, using `tools/sync-translations` (which invokes `tx
+   pull`, internally).
 
 ## Translators' workflow
 

tools/sync-translations

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+
+set -e
+set -x
+
+./manage.py makemessages
+tx pull -a -f --mode=onlytranslated --minimum-perc=5
+./manage.py compilemessages