Fix whitespace issues and add EditorConfig file

I ran an experimental style checker against the code and identified some mixed indentation, trailing whitespace, and missing final newlines.

Changes I made:

- Trim trailing whitespace from Python, SCSS, and JS files
- Convert all SCSS files to tabbed indentation (some were used spaces or mixed indentation)
- Insert final newlines that were missing from many files
- Add an `.editorconfig` file for enforcing the correct code styles

In the  EditorConfig file, I allowed trailing whitespace in the HTML files, and added some (possibly unnecessary) rules for the font-awesome and other third-party library files.  I can remove the third-party lib rules, trim trailing spaces from HTML files, or make other changes upon request.

You can verify that only whitespace changes were made by viewing the diff with [?w=1 in the url](https://github.com/django/djangoproject.com/pull/273/files?w=1) or by running `git diff master -w`.