search: more server-search refinement

fixed bugs, added extra text on "no results found" to suggest server
searching, and conditionally disable the button based on search content
and current sort. Activities page still broken. Also fixed up cache
generation, only one should ever run now, as should sorting. Two time
thresholds exist, one to trigger a re-sync but do it in the background
(i.e. send the old one to the requester), and one to re-sync and wait
for it.

css/tooltip.css

@@ -27,6 +27,12 @@
     right: 0;
 }
 
+.tooltip.above .content {
+    bottom: 2.5rem;
+    left: 0;
+    right: 0;
+}
+
 .tooltip.darker .content {
     background-color: rgba(0, 0, 0, 0.8);
 }