Fix a bunch of stuff

New compatibility:

• Support for Django 2.0+ (#247)
• Support for jQuery 3+ (#197, #233, #224)
• Add compatibility with django-crispy-forms (#229)
• Add compatibility with django-js (#239)

Fixes:

• Adding formsets (#211)
• Update all .chained fields (#212, #222, #223)
• Fix rendering when horizontal=True (#225)
• Workaround a JS bug in Django (#246)

Changes:

• Corrected documentation - remove SMART_SELECTS_ prefixes from some settings

Almost all of this release is built from pull requests - I did barely any coding at all. Thank you to:

• @vborras
• @silentjay, @erozqba, @oceannw, @jbnance
• @ChloeStevovitch
• @PetrDlouhy
• @leibowitz
• @CloudVE and @nuwang