This allows for templates using {% if article.modified %}
instead of {% if article.modified != article.published %} .

added sourcecode to the functional test so it's tested *somewhere*.
fixes #963

Deliberate overriding via `save_as` metadata should be allowed, even after the
overwrite detection feature. This commit is to add tests for deliberate
overriding. As a result, the relevant tests *should fail* after this commit.

Added a page and an article, both to override a tag, with very old dates so
it limits the amount of diff in the generated pages.

Overriding feature introduced by d0e9c52410e70e14008d2ef827ba7ac306243e72
Overwrite detection introduced by ff7410ce2ada85b486a67ae11874d60d135ff939

I'd added them earlier to test that a configuration edit could
preserve the original output locations.  However, it is likely that
you have quite a number of static files, and we shouldn't recommend
listing explicit paths for all of them.  With this configuration
change, the pictures will be copied into the output directory using
their original relative path (e.g. `pictures/Fat_Cat.jpg` without the
`static`).  Any |filename|-style links will be updated automatically.

If you *want* the pictures to end up in a `static` directory, it's
easier to just organize your source that way.

and update functional tests output

closes #614: cannot reproduce this issue.

and update functional tests output