ENH Avoid redundant CSS in Styler.render
Where multiple styled cells have the same CSS, only make one CSS declaration. This can reduce the output size substantially.

- [x] ~~closes #xxxx~~
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
