Try: improve hierarchical display of category parent select

[adamsilverstein]

Adding a visual Hierarchy

The category parent selector currently indicates the parent/child relationship in the dropdown by indenting entries. Actual indentation levels are difficult to discern. Now that select elements can be fully styled HTML, this PR aims to improve the select with a graphical treatment that leverages CSS to display the "tree" structure of the hierarchy.

Here is what I have so far:

Before:

(this is how the select looks in non-supporting browsers as well)

After:

(could use a little polish to remove extra lines and reduce indents slightly)

Trac ticket:

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[github-actions]

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