Add tagging for cards(, question types, question views)

[NiloCK]

Scope:

This issue is for tracking work on the application-logic of the tagging system. For the UI / discoverability of tags, see #143

Content:

A card should be taggable with metadata in order to facilitate relational inferences among cards, and in order to catalog content against, eg, a curriculum standard. A draft:

class Tag {
  text: string // the text of the tag itself
  descriptorText: string // markdown string of text that elaborates on the tag, possibly containing links to source material
  parent(s?): Tag // parents get auto-added when a tag is added. Eg, add common-core.math.9.number-sense-1.2, and common-core.math.9 is added automatically
}

Tags will also serve in order to create filtered study sessions: eg, audio free, calculator required / forbidden, pencil & paper / mental math.

to do:

• update above to current implementation / revisit above idea(s) (parent -> child card relationship, especially)
• tagging during card creation
  • BUG: tag on create is applying tags to the created note, not to the cards generated from that note.
  • fix this bug
• tag suggestion engine?
• system-tags - non-human generated tagging information should not be visible - how to distinguish
• filter-function tags
  • implementation
  • UI to distinguish them from 'label' tags
• tagging during a "browse" session instead of during "study"
• course tags browsing: Expanded in /course/{id} - work tracker #143
  • listing, eg, card count w/ this tag. browsing cards under the tag
  • editing metadata for tags (short explanatory blurb, longer-form md 'wiki')