bootstrap: BS5 sweep follow-ups (form-group, drop popper.js@1)

Post-swap hardening after a full BS4-removed-class sweep of the codebase:
- LocationAutocomplete passed `root: 'form-group'` to react-places-autocomplete;
  BS5 removed `.form-group` (it gave the bottom margin) -> use `mb-3`.
- Drop the unused `popper.js@1` dependency (react-bootstrap bundles
  @popperjs/core@2). No source imports it.

Sweep otherwise clean: no other BS4-only utility/component classes remain in JSX
or SCSS. Full-app build boots clean.

imports/ui/components/LocationAutocomplete/LocationAutocomplete.js

@@ -88,7 +88,7 @@ class LocationAutocomplete extends React.Component {
               styles={myStyles}
               autocompleteItem={AutocompleteItem}
               classNames={{
-                root: 'form-group',
+                root: 'mb-3', // was BS4 `.form-group` (removed in BS5); mb-3 keeps the bottom margin
                 input: 'form-control',
                 autocompleteContainer: 'autocomplete-container'
               }}