Fix mobile layout: wrap unresponsive tables, let the group filter bar wrap (#42)

Auditing mobile/responsive views turned up two classes of bug:

1. Missing .table-responsive wrapper (token.ejs and profile.ejs's other
   table already had it): admin.ejs (2 tables), invites.ejs (6 columns),
   notifications.ejs (5 columns), users.ejs (7 columns — the worst case).
   Without it these overflow the page horizontally on narrow viewports
   instead of scrolling within the table.

2. groups.ejs's search/sort/count toolbar was a plain `d-flex` (no
   flex-wrap) containing a search input, a <select> with a hard
   min-width:175px, and a nowrap count span — on a narrow viewport
   there's nowhere for that content to go but off-screen. Added
   flex-wrap and gave the search input a flex-basis so it wraps onto
   its own line first when space is tight.

Verified: EJS compiles for all five templates, npm test 192/192 pass,
and fetched each route from a running instance to confirm the fixes
are present in the served HTML.

nodejs/views/invites.ejs

@@ -246,6 +246,7 @@
 					</div>
 					<div class="card-header actionMessage" style="display:none;"></div>
 					<div class="card-body p-0">
+						<div class="table-responsive">
 						<table class="table table-sm mb-0">
 							<thead>
 								<tr>
@@ -261,6 +262,7 @@
 								<tr><td colspan="6" class="text-center text-muted py-3">Loading…</td></tr>
 							</tbody>
 						</table>
+						</div>
 					</div>
 				</div>
 			</div>