fix JS console errors

[simonLeary42]

Closes #382

• bug 1: filter.js, sort.js are loaded on all pages, but they assume that a table will always exist
• bug 2: in remove cache from big pages #266 I added a nested table for user groups in user-mgmt.php, but did not update the query selector for table rows to exclude the nested table rows
  • this is done by querying :scope > tr on the <tbody> element
• bug 3: in use <th> instead of <tr class='key'> #437 I replaced <tr class='key'> with <th>, but did not update the query selector lines to look for the new element type
• also wrapped stuff in an immediately invoked function expression IIFE to avoid future variable name conflicts

[Copilot]

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