Archive database persistence and indicator/selector #843
Description
The dbname= QS parameter to use an alternate TM database instance is currently not persistent and can only be specified by hand-editing a URL.
In https://forum.travelmapping.net/index.php?topic=7089.msg38813#msg38813 @michihdeu has a suggestion of a yellow banner indicating the currently-selected archive database. Combine this with a way to select among the archives and we have something workable.