fix(frontend): fix and enhance candy modal tags input box

enhancements to the candy modal tags input box:
* unify look and feel with remaining of form
* support for custom tags input validation
* new custom tags validation directive with unit tests
* support disabling/enabling Candy Modal Save button based on validation

Closes #12
Merge pull request #26 from ghachey/issue12

fix(frontend): fix and enhance candy modal tags input box