Button styles and icon implementation updates

[Derick]

Hello,

We have some changes to our button styling, as well as how we plan to implement
icons. The review is
https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-webui/+/22396.
It would be great to get some feedback on the implementation! This
change will allow us to
control icons using CSS and start to remove all the duplicates needed
to have icons
of different colors.

Thanks!
Derick Montague