Fixed encoding error in built HTML file timestamp

* Now, building games in non-English locales should no longer get an
encoding error caused by the built file's timestamp. Thanks to @mthuurne
for assistance and Cab for the report.
* Also reverted the "New Passage creates missing Start passage"
behaviour, having noticed that it would be irritating for "partial .tws
files" that are being used in a large StoryIncludes project.