Override exported content node duration with completion criteria duration

[bjester]

Desired behavior

• If a node's completion criteria has a model of either time or approx_time, the exported ContentNode created during channel publishing should have the threshold written to the duration field.
• If the node is audio or video, the completion criteria threshold should override the file duration, for the above situation, and otherwise should contain the actual file's duration (current behavior).

Current behavior

• Duration is exported if they're audio or video and contain this information in the database
• Completion criteria is exported to the options
• No overriding occurs

Value add

• Kolibri expects this to occur in this manner

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