Open a story in the component Playground

[cblavier]

Currently, the component playground is initialized from the first Story declared in the component entry file (or, if the first item is a group, from the first story in the first group).

The idea would be to keep this behavior as a default, but to be also able to open the component playground with any story:

• in the component Stories tab, there is a link below each story to open it in the playground
• in the component Playground tab, there is a select input below the attributes tab to pick another story
• the story id the playground has been opened with must be reflected in the URL (?story_id=my_story)

Handling of StoryGroup is not part of this issue; it will be described in another ticket.