accounts: infinite scroll for performance

Found out the bottleneck when ~2000 or more elements are loaded isn't
the search or sort or anything, but the DOM. An infinite scroll
implementation is added, where elements are added to the DOM as you
scroll. May still be a little buggy, and can't yet cope with screen
resizes. Also, the "shown" indicator is broken.

config/config-base.yaml

@@ -65,6 +65,20 @@ sections:
     type: number
     value: 30
     description: Timeout of user cache in minutes. Set to 0 to disable.
+  - setting: web_cache_async_timeout
+    name: User search cache asynchronous timeout (minutes)
+    requires_restart: true
+    advanced: true
+    type: number
+    value: 1
+    description: "Synchronise after cache is this old, but don't wait for it: The accounts tab will load quickly but show old results until the next request."
+  - setting: web_cache_sync_timeout
+    name: User search cache synchronous timeout (minutes)
+    requires_restart: true
+    advanced: true
+    type: number
+    value: 10
+    description: "Synchronise after cache is this old, and wait for it: The accounts tab may take a little longer to load while it does."
   - setting: type
     name: Server type
     requires_restart: true