Filters should apply safe-ness rules to filter arguments

[Stephen Kelly]

This was reported to security@, but is being treated as a normal bug because
it requires lots of prerequisite knowledge from an attacker.

A filter uses is_safe=True to note that if given safe input it will generate
safe output. However, it seems that only applies to the 'value' argument to
the filter, but not to the 'arg' argument.

The attached patch and test to the join filter may make this clear. Currently
string literals and variables are rendered unescaped, so join05 and join07
fail.

This could be exploitable if an attacker knew the join filter was used join
arguments with another user-supplied argument. I have not looked extensively
yet at other filters, but there could be others vulnerable to this.

[Jacob]

(In [13464]) Fixed #11377: the template join filter now correctly escapes the joiner, too.

Thanks, Stephen Kelly.

[Jacob]

(In [13465]) [1.2.X] Fixed #11377: the template join filter now correctly escapes the joiner, too.

Thanks, Stephen Kelly.

Backport of [13464] from trunk.