taskboard: added row-folding to expand and collapse row; show notes-icon if card has notes

- taskboard-row:
  - fold row with click on '+' or '-' icon on left row-menu
  - collapsing row hides cards except in first column
  - change "folded" background-image on collapsed row
  - row-fold-state is stored in database and sync'ed to other users

- taskboard-card: show notes-icon on small card for zoom-level 0 & 1, if card has stored notes shown on opening big card view

- added screenshots of some new functionality of taskboard
taskboard-card: added colors brown/blue/magenta in card-color-picker

- example usage: technical debt (brown), impediments (blue), magenta (evaluation-bug/task)