fix(develop): fix links to embedded forms in dev sample
feat(tasklist/forms): upgrade dev forms to latest features

Update our test task forms to use some of the new validation features

- invoice/approve.html: add ng-required
- invoice/assign-approver.html: add ng-pattern

Related to CAM-1322
feat(tasklist/forms): add input extensions (form-field)

Rewrite the compilation of form fields in embedded task forms to
allow extensibility of form-field annotated form elements:

- All validation attributes such as ng-pattern, required, etc. get
  properly added to the resulting form input. This allows custom
  validation and pattern matching to take place.

- Outer form is exposed as `variablesForm` to the current scope.

- Validation state may be queried via
  variablesForm.{inputName}.($valid|$invalid), i.e. the following
  markup in an embedded form will show a validation error to the
  user unless `test` is put in:

  <input form-field type="string"
         required ng-pattern="/^test$/" name="testForm" />

  <p ng-if="variablesForm.testForm.$invalid">
    Need to type in <em>demo</em>.
  </p>

- Form submits only if validation state is $valid

Related to CAM-1322
Merge branch 'master' of github.com:camunda/camunda-bpm-platform