M::M::U: Store graph categories in SQL in lowercase

As discussed on IRC, graph categories should be case insensitive.
cgi/html: Implement basic category view

warning/critical state and graphs without explicit categories are
not supported yet.
M::M::U: Export plugin/field directives for custom graphs to SQL
cgi/html: Fix URLs for comparison views
Merge remote-tracking branch 'upstream/master' into cgi-html-fixes-to-match-static-html

Conflicts:
	master/lib/Munin/Master/Update.pm
cgi/html: group comparison view
Merge remote branch 'pull/230' into devel