ts_only search parameter

There is now no separate check box "Search only for stations with time
series"; instead, "ts_only:" can be specified in the search box.
Add help for advanced search; remove old search

Removed old search form; now we search only from the toolbar. Added
"search tips" item on the toolbar that pops up search help.
Improve "list maps visible on map" button

Converted "list maps visible on map" button to link; improved its
wording; improved its JavaScript; and removed some obsolete JavaScript.