Add ability to close Story panel (other than by clicking the Story button again)

[chloeleichen]

#3537

Suggested solution

[chloeleichen]

DO NOT MERGE, FURTHER DESIGN NEEDED

[AnaBelgun]

@philipgrimmett ping for refinement of design

[philipgrimmett]

Invision prototype with 'hide panel' button for data stories editor:
https://invis.io/43RSPX2U5JD#/360676208_story_-_Panel_Open

[zoran995]

Should this be closed, it is implemented in both master and mobx

[CLAassistant]

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