Use the new json function in INGInious templates

This line will fail if there is a tag into the json:
https://github.com/Maxmawt/INGInious-problems-blockly/blob/master/inginious-problems-blockly/templates/box_blockly.html#L10

Let's fix it using the new json() function introduced by https://github.com/UCL-INGI/INGInious/pull/337
(wait until the PR is merged, of course ;-))

Inspect templates/box_blockly.html around line 10 and read the referenced INGInious PR introducing json(). Update the template's JSON handling there, then verify that data containing a tag renders safely without breaking the template.

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