use a separate (memory-based) cache for sessions
don't let topic tree collapse when it's empty; always display 'None' for 'selected topics' when empty
hide the form element for adding a group to a teacher, also disable :)
previous checks on whether caching is enabled now fail.  Fixing to check CACHE_TIME instead.
sessions cache uses default cache only.  webpage cache can use named cache. so, migrate to this.
moving topictree code to d3_viz template.  make 'none' text unclickable, show in red
Merge branch 'develop' of github.com:learningequality/ka-lite into topictree_collapse

Conflicts:
	kalite/templates/coachreports/base.html
Merge pull request #402 from bcipolli/topictree_collapse

scatter/timeline coach reports: don't let topic tree collapse when it's empty
Merge pull request #339 from bcipolli/separate_sessions_cache

use a separate (memory-based) cache for sessions