Permalink to focus mode on board
From https://gitlab.com/gitlab-org/gitlab-ce/issues/39459, as suggested by @brendan
If the "fullscreen" button actually links to the permalink itself I think that makes the most sense. That where there is no difference between the clicking the button or visiting the permalink.
And as a user, if I bookmark the board in "full screen", I then automatically come back to full screen when I click that bookmark.
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