Make XWiki responsive to font size user preference

There was an interesting talk about accessibility at FOSDEM that argued that websites should respect the user's settings in the browser instead of introducing new controls. There was an explicit question at the end if it would be bad for a web application to introduce a font size control and the presenter kind of agreed that it would be better to just respect the font size that the user has configured in the web browser. Also, all web browsers allow changing the font size/zooming the website. Thinking about it again, I wonder if it wouldn't really be better to just respect what the user set in the browser's settings and make sure that the browser's zoom feature works properly in XWiki.

from https://github.com/xwiki/xwiki-platform/pull/2694#issuecomment-1927063988

In my opinion we can have both systems.

The improvement this issue targets is the responsivity of the UI to the browser level user preference about font-size.

See this forum discussion about this improvement: https://forum.xwiki.org/t/deprecating-the-font-size-base-colortheme-variable/14005