Set pristine forms on auth reset

Relates to #1200
Improve slow-validate directive and usage

Respond to view value changes by watching the view value on the scope
rather than registering a view change listener. View change listeners
only fire on changes which are valid.

Be careful not to use the directive in combination with ng-hide or
ng-show since the CSS changes subvert the transition.

Improve the conditions for showing/hiding so that it considers the
validity of a field in addition to whether the field is pristine or
dirty.

Relates to #1200
Reset auth sheet forms on close

Relates to #1200
Propagate form errors to caller and flash for auth

Relates to #1200
Merge pull request #1204 from hypothesis/1200-auth-fixes

1200 auth fixes