Reasons for creating this PR

The rendering of the concept hierarchy was very slow in some cases, see #1377. The reason is that HTML code is being parsed within the nodeLabelSortKey function and this can be very slow when there are many concepts shown in the hierarchy. This PR changes the way nodeLabelSortKey is implemented to avoid the slow HTML parsing, thus making the hierarchy show up a lot faster.

I couldn't find an easy way to benchmark this but the speedup is quite dramatic for instance in the case of the YSO concept "haircutting" that was used as an example in #1377.

Link to relevant issue(s), if any

• Closes Opening the hierarchy for a concept takes a long time #1377

Description of the changes in this PR

• attach the labels to jsTree nodes representing concepts as a separate attribute label (similar to e.g. uri and notation)
• when sorting nodes, look at the label attribute instead of parsing the HTML code to find it
• remove extra trailing whitespace from a few lines that are directly adjacent to the above changes

Known problems or uncertainties in this PR

This PR changes the functions createConceptObject, createObjectsFromChildren, vocabRoot and createObjectsFromNarrowers which are also overridden in the footer.inc file which is part of the Finto customizations. If you have installed the customizations as they are currently written, this code will not work properly because the label attribute will be missing from nodes. The changes need to be applied to footer.inc as well. I will do that shortly.

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)