Fix bad query terms with nested subjects

Reported by @patrickmj. Subjects like "Catholicism--Converts" had bad
query terms because the "--" was replaced with nothing. Now it is
replaced with a space. This sometimes creates lots of extraneous spaces,
but that shouldn't affect most catalogs.
Merge branch 'hotfix-nestedsubjects'
Merged bugfix for nested subjects
Changelog added
Merge branch 'master' into develop

Bring in CHANGELOG
Add note about multiple subjects
Display only catalog searches that should be displayed
Decide which query terms to use