When investigating topic containers for mobile (T303362), we've noticed that the styling of section headings is currently implemented using `display: table` and `display: table-cell`, which makes it difficult to add content associated with the heading that would display on a separate line. It seems like they could be implemented using `display: flex` instead, which was not widely supported by browsers when this code was originally written, but it is now. A flexbox version would be easier to maintain in the long term, and it would also allow us to remove some workarounds in VE code (e.g. T202991).
===Testing
This should result in no changes (regressions) to the look and behaviour of section headings on mobile, which usually consist of:
* An expand/collapse icon on the left (^/v), clicking on this or the heading should toggle the section
* The section title
* And section edit link (pencil icon), right aligned, which launches an editor at that section
Test on a few mobile devices (Android/iOS)