#144208805 Feature: Export Workouts.

[paulupendo]

What does this PR do?

Enables users to export their workouts in json format. These json data can then be imported by other users.

Description of Task to be completed?

Have the following to implement the feature:

Add a button in the workout options dropdown menu to allow access to the feature
Create a form to allow users to comment on the export
Ensure returned response from user request is a download of the workout in json format

How should this be manually tested?

• Choose the workout option on wger.
• Click on any of your available workout plans
• On the far right hand corner of the redirected page click on the options tab
• Choose the export workout option in the dropdown menu
• You will be directed to an export workout form
• Click on the export workout button to download workout in json format.

What are the relevant pivotal tracker stories?

PT story ID: #144208805