WEB: Added theme dependent favicon 
- [x] closes #42314
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Added a light mode version of the favicon based upon the .svgs in the static folder as well as added a small js scipt to the template page to detect the theme change.

![mgCmqcreiy](https://user-images.githubusercontent.com/53790060/123965934-fe1c8a00-d9c5-11eb-8838-ce72ee988a3c.gif)

cc @datapythonista if any comment @datapythonista I probably have done it this way in the first place, but I've made the changes you've requested, now updates the favicon without requiring refresh through use of a event listener. And now just updates the href of the current favicon. Definitely a lot simpler. 
![363GRl3Xq6](https://user-images.githubusercontent.com/53790060/124248105-91270280-db33-11eb-804c-2aa41866ed93.gif)
 Are there any more changes that need to be made? Or can it be safely merged without conflicts? Sorry for the delay @DeviousLab. We usually require two core devs to review a PR before merging, let's wait a bit more and see if someone else can have a look. Thanks for your patience. > Sorry for the delay @DeviousLab. We usually require two core devs to review a PR before merging, let's wait a bit more and see if someone else can have a look. Thanks for your patience.

No worries! just thought I might have been missing something @jreback Does this look good to you? thanks @DeviousLab 