[ENG-5318] Add is-mobile helper

[futa-ikeda]

• Ticket: ENG-5318
• Feature flag: n/a

Purpose

• Add a new helper to avoid having to make unneeded component.ts and controller.ts files

Summary of Changes

• Create new is-mobile helper
• Test for new helper
• Not currently implemented in actual pages

Screenshot(s)

• NA

Side Effects

• None

QA Notes

• No QA needed as nothing functional is changing

[futa-ikeda]

Hmm, this actually seems redundant based on what is in the ember-responsive docs

{{#if (media 'isDesktop')}}
  Desktop view!
{{/if}}

[cslzchen]

LGTM, with a couple of comments/questions