Avoid translating lines with ;

This is a very simple work-around for us extracting lots of code to
the `messages.pot` file. The comments in the code can be translated,
but Cloud Translate doesn’t know about it: instead it translates
everything, keywords and all.

The course text uses `;` only very sparingly: I found a single page
which uses the character. So we are not losing much by skipping these
messages.

Long-term, we should extract code blocks at a unit and we should mark
them in the `messages.pot` file (google#318). That will allow us to be more
selective about what we translate.

cloud-translate/src/main.rs

@@ -137,7 +137,7 @@ async fn translate_catalog(
             continue;
         }
         let msgid = msg.get_msgid().unwrap();
-        if msgid.trim().is_empty() || msgid.contains("```") {
+        if msgid.trim().is_empty() || msgid.contains("```") || msgid.contains(";") {
             continue;
         }
         char_count += msgid.chars().count();