Often times, it can be confusing to set a custom permalink structure. One has to double-check the documentation, make
sure to correctly insert the structure tag, and hope not to break their site.
With this addition, the available structure tags are being displayed as a list of easily clickable buttons that can be
used to insert tags to the custom structure input field and to remove them again.
Props kpdesign, swissspidy, joedolson, afercia.
Fixes#29872.
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