adapt-contrib-vanilla - Update media play image to be an icon

[guywillis]

Subject of the issue

The media play icon is built using a background image sprite with the hover state created by offsetting the background position. This solution isn't the easiest to maintain or customise.

Current implementation:

Your environment

• v5 Framework

Proposed solution

Recreate the current implementation but use the newly created video play icon, convert the positioning CSS to use relative values instead of px, and add an opacity transition between default and hover states.

Mock of new implementation:

Dependant on #2688