Code style issues (mostly whitespace inconsistencies)

I noticed many whitespace inconsistencies, some of which confused my text editor while hacking on django-grappelli.

Most of the problems were with HTML files:

* Some files used LF line endings, some used CRLF, and some used a mix of both.
* Some files used spaces for indentation while some used tabs
* Some files had a trailing newline and some didn't


There are also multiple Python files that have trailing whitespaces.  I fixed a few, but I couldn't tell whether some of the dashboard files were like this intentionally.

I used a `.editorconfig` file and an EditorConfig file checker to try to fix these problems in a semi-automated way, but I haven't figured out what the style *should* be in each file (tabs vs. spaces for example) and which files to exclude from checking.

I may turn this into a pull request if this is something you think is worth fixing and you can give some feedback on what the format should be.  Here is an example `.editorconfig` file I was trying to use to locate the inconsistencies:

    [*.{py,html,js,scss}]
    indent_style = space
    indent_size = 4
    end_of_line = lf
    trim_trailing_whitespace = true
    insert_final_newline = true

    [*.scss]
    indent_style = tab