The changes in 948634fe31 meant that only the most-recently-changed placeholder is actually applied.
This commit refactors things so that we store all placeholder values in JS, and then apply them all in a single pass over the DOM. As well as fixing the bug, this should be a significant perf improvement for posts with lots of placeholders
Also introduces some simple system specs.
---------
Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>
David Taylor