Munge text to make i18n singular forms unique.

Summary:
For ngettext strings ("1 test"/"{{num}} tests"), our translation tools
get upset if we see the same singular text in two places, but with two
different plural texts ("1 test"/"{{num}} tests") vs ("1
test"/"%(num)s tests").

There were two places this happened in khan-exercises, one because
a weird line-breaking choice made two plurals look different that were
actually the same, and the other because of a distinct varname.  We
solve the second in the normal way of replacing '1' with 'varname', so
that the singular forms now differ.

Task T1562

Test Plan:
Ran
   tools/pybabel extract -F tools/babel.ini khan-exercises
without errors.

Reviewers: james, eater, alpert

Reviewed By: alpert

CC: alpert

Maniphest Tasks: T1562

Differential Revision: http://phabricator.khanacademy.org/D4139

exercises/area_1.html

@@ -149,8 +149,8 @@
                         <span data-if="isSingular(L)">
                             The length is <var>L</var> <var>UNIT_TEXT</var>.
                         </span><span data-else="">
-                            The length is <var>L</var> <var>plural_form(UNIT_TEXT, L)
-                        </var>.</span>
+                            The length is <var>L</var> <var>plural_form(UNIT_TEXT, L)</var>.
+                        </span>
                         <span data-if="isSingular(W)">
                             The width is <var>W</var> <var>UNIT_TEXT</var>.
                         </span><span data-else="">

exercises/decimals_on_the_number_line_1.html

@@ -80,7 +80,7 @@
                 <p><code>\pink{<var>SOLN_TXT</var>} = \purple{<var>MARK_INCREMENT</var>} \times \green{<var>TENTHS</var>}</code></p>
 
                 <p data-if="isSingular(TENTHS)">
-                    Therefore, moving the orange dot <code>\green{1}</code>
+                    Therefore, moving the orange dot <code>\green{<var>TENTHS</var>}</code>
                     place will put it at the position <code>\pink{<var>SOLN_TXT</var>}</code>.
                 </p>
                 <p data-else="">

exercises/decimals_on_the_number_line_2.html

@@ -81,7 +81,7 @@
                 </code></p>
 
                 <p data-if="isSingular(HUNDRETHS)">
-                    Therefore, moving the orange dot <code>\green{1}</code>
+                    Therefore, moving the orange dot <code>\green{<var>HUNDRETHS</var>}</code>
                     place will put it at the position <code>\pink{<var>SOLN_TXT</var>}</code>.
                 </p>
                 <p data-else="">