Ignore JetBrains project folders. (rebased onto develop)



This is the same as gh-2343 but rebased onto develop.

----

This minuscule change makes Git ignore ``.idea`` folders created when working with the codebase in JetBrains IDEs (PyCharm or IntelliJ IDEA). To test, open a part of the codebase in one of those IDEs and verify that ``git status`` doesn't show new untracked folders named ``.idea``.