Fix TypeError in journalist.js

The quick filter dialog box was being checked on document load, even if
we were on pages (e.g. source collection pages) that don't have that
dialog box. This caused a TypeError, when we tried to match against an
undefined value from the non-existent dialog box.

Fixed by checking that the dialog box exists before calling
filter_codenames.
Merge pull request #599 from freedomofpress/fix-journalist-js-typeerror

Fix TypeError in journalist.js