Avoid deleting previous PYTHONPATH

With the previous putenv command, if an user had already set the PYTHONPATH
environment variable, its value was deleted and substituted by os.getcwd(),
which may prevent the installation if the dependencies are installed in a
different path. Now, it appends this value to the previous value set in the
sys.path, so the user configuration is kept.

Reviewed at https://reviews.reviewboard.org/r/6001/
Merge branch 'release-0.8.x'
Fix a couple style issues in config-forms.less.

This changes one of the rules to use '.box-title' instead of 'h3',
for consistency with other rules.

It also changes the subsection styles to match what we had in RBCommons.
Nothing but RBCommons seems to use this style currently, and I don't
know why the style differed from what we had there, but worth moving it
back.

Reviewed at https://reviews.reviewboard.org/r/6004/
Merge branch 'release-0.8.x'