Ensure incomplete tags aren't lost on save

closes #2991, references #2172, references #2453
- moved and separated tags logic from EditorTagsView into PostTagsInputController/View
- call out to PostTagsInputController when saving post to ensure an incomplete tag is completed before save
- added Tab key support for suggestion selection / tag completion
- don't show suggestions list when input field doesn't have focus
- added code for #2172 but left commented-out as it causes side effects with completion on save
- updated suggestion highlighting so it doesn't bork on html/regex chars (#2453)