alias settings.DOCSTORE_INDEX to mounted device; use for searching

We want to have separate Elasticsearch indexes for each storage
device, but we want to be able to access the currently mounted
device by a single alias name.
This change sets the settings.DOCSTORE_INDEX alias when mounting
a device or path, and uses only DOCSTORE_INDEX for searching.
indicate target of DOCSTORE_INDEX alias on search pages
Display "Select an index" message for search-admin menus

This instead of menus with the first index selected.
input sanitization
don't append 'ddr/' directory if path already ends in 'ddr/'
added some logging points to mount functions
Merge branch '171-docstore-index-alias'
better error msgs for search when storage devices not available
better input/output docs for webui.set_docstore_index