Hints in assignment editor

[torgeirl]

The assignment editor support some HTML tags such as <details> and its <summary> that some assignment courses have used to add hints with. These lack ARIA attributes and don't seem to be accessible to all users. We should probably add a better mechanism for hints, that work for all user agents. Edit: since <details> and its <summary> are native HTML elements they shouldn't require ARIA attributes.

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Related: some of these hints are styled with <u> tags, creating underlined elements that aren't links but act like it by toggling the non-summary text of the hint on/off. We should probably make these work in a better way to eliminate hacky workarounds like that.