* Use a single capability 'mod/workshop:exportsubmissions' to control
who is able to export a submission and all associated visible
peer-assessments.
* The export capability is by default granted to teachers and managers
as well.
* Simplify the integration of the export button into the submission
page. There is now a single place where the export button appears - at
the buttom of the submission page.
* Clean up the new strings introduced for this feature.
* Rewrite the portfolio caller class so that it better mimics the
behaviour of the submission.php page. It must not be possible to get
access to contents otherwise not available via the UI.
* Fix phpunit tests.
* Improve behat tests so that they can run in headless browser
(performance).
During the development, some problems were detected in the underlying
portfolio API, especially with LEAP2A format and files collisions
handling. These may be eventually fixed in the future, should there be a
demand for it.
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