Renamed to Tagr to Mindtagger
Mindtagger refactoring: moved most data operations to backend

- Now supports multiple Mindtagger tasks via passing many mindtagger.conf files to mindbender-tagger command.
  - mindtagger.conf specifies
    - how to get items (currently supports only a single tsv/csv/json
      file)
    - what presets to use with what options(!)
  - The directory where mindtagger.conf sits will be used as a workspace
    for Mindtagger, holding the tags.json and any other artifacts.
- Moved schema derivation to backend.
- Moved most of the preset mixin mechanism to frontend.
Mindtagger moved buttons to navbar-fixed-top
Mindtagger redirects to the first task
Removed data-toggle="dropdown" as it's handled by ui.bootstrap
Mindtagger halt frontend when committing a tag fails
Mindtagger word-array: support styling of words indexed by different columns

whose values are serialized postgres arrays.
Mindtagger context for showing surrounding sentences
Renamed precision-tags to is_correct
Mindtagger custom preset functionality fixed with isolated namespace
Cleaned up log messages and fixed a mindtagger POST bug