Babel underscore extraction update according to @nedbat

1. Add underscore.po into the merge section in config.yaml
2. Using one babel command template to construct two babel commands

conf/locale/config.yaml

@@ -108,6 +108,9 @@ segment:
     mako.po:
         mako-studio.po:
             - cms/*
+    underscore.po:
+        underscore-studio.po:
+            - cms/*
 
 # How should the generate step merge files?
 generate_merge:
@@ -122,3 +125,4 @@ generate_merge:
         - djangojs-partial.po
         - djangojs-studio.po
         - underscore.po
+        - underscore-studio.po

i18n/extract.py

@@ -63,16 +63,16 @@ def main(verbosity=1):
     else:
         stderr = DEVNULL
 
-    babel_mako_cmd = 'pybabel {verbosity} extract -F {config} -c "Translators:" . -o {output}'
-    babel_mako_cmd = babel_mako_cmd.format(
+    babel_cmd_template = 'pybabel {verbosity} extract -F {config} -c "Translators:" . -o {output}'
+
+    babel_mako_cmd = babel_cmd_template.format(
         verbosity=babel_verbosity,
         config=base(LOCALE_DIR, 'babel_mako.cfg'),
         output=base(CONFIGURATION.source_messages_dir, 'mako.po'),
     )
     execute(babel_mako_cmd, working_directory=BASE_DIR, stderr=stderr)
 
-    babel_underscore_cmd = 'pybabel {verbosity} extract -F {config} -c "Translators:" . -o {output}'
-    babel_underscore_cmd = babel_underscore_cmd.format(
+    babel_underscore_cmd = babel_cmd_template.format(
         verbosity=babel_verbosity,
         config=base(LOCALE_DIR, 'babel_underscore.cfg'),
         output=base(CONFIGURATION.source_messages_dir, 'underscore.po'),
@@ -108,8 +108,6 @@ def main(verbosity=1):
 
     files_to_clean = set()
 
-    files_to_clean.add(source_msgs_dir / "underscore.po")
-
     # Extract strings from third-party applications.
     for app_name in CONFIGURATION.third_party:
         # Import the app to find out where it is.  Then use pybabel to extract