Support browser font scaling

[kerryarchibald]

Fixes https://github.com/vector-im/element-web/issues/26669

• sets html font size to 100%, and relative to 100% where baseFontSizeV2 is not == default. This change means EW will now respect browser font sizes and scale text accordingly.
• removes useCustomFontSize setting and input
• reinstates displayFunc on Slider component
• displays font slider values as relative to default ("+2")

Checklist

• [ ] Tests written for new code (and old code if feasible)
• [ ] Linter and other CI checks pass
• [ ] Sign-off given on the changes (see CONTRIBUTING.md)

---

Here's what your changelog entry will look like:

✨ Features

• Support browser font scaling (#11972). Fixes vector-im/element-web#26669. Contributed by @kerryarchibald.