avoid "ember-cli-page-object.string-properties-on-definitio" deprecation

[ro0gr]

see http://ember-cli-page-object.js.org/docs/v1.17.x/deprecations.html#string-properties-on-definition

@mixonic I've assumed the problem described in san650/ember-cli-page-object#534 should have been somehow appeared in here, but instead I've noticed this deprecation on CI.

From the ec-page-object standpoint, the deprecation for the plain strings on definitions is a part of a plan to enable a more fluent way to write definitions. So at some point, any plain strings should be treated as a scope of nested component(san650/ember-cli-page-object#408).

Wrapping activeClass with a getter, allows to hide a string from the ec-page-object, and it fixes the deprecation warning.

Just in case, there is another possible direction to fix the deprecation, like:

 isActive: hasClass('is-active', '[data-test-toggle-button]');

But seems the point of this page object is to demonstrate/test native get support for the isActive, so I went with adding another getter to the page object.

---

I did also notice some other deprecations caused by outdated usage of ember deprecate( utility by ec-page-object. I'll try to fix it , and publish new version this weekend.

[mixonic]

@ro0gr ah thanks for this! I was literally just looking at this and wrote the same patch! :-)