Add model and views for a workout session

A workout session is an actually performed training session that allows
the user to leave impressions and any notes.
Expose workout sessions through the REST API and add tests
Add workout session form to the weight log entry page
Add uniqueness constraints and validators to workout sessions
Increase tested maximal filesize length for nutritional plan PDF

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extra : source : 7aa7fa12a02b4d44f123ea5922737097cfc9ca67
Update changelog
Show the workout session in the calendar view
Allow to add a workout session directly from the timer view
Improve rendering of workout session form in gym view
Fix tests for weight log
Create or add a new workout session from the gym mode page

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extra : amend_source : 3259e087063709d5424f7c46d4bfd845bfa42daa
Automatically update existing workout sessions if available
Allow editing of workout sessions from calendar view
Don't create users when accessing the REST API

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extra : amend_source : a43c1f688cbc0304bdf545349699ea33086ec1ef
Update help text for workout sessions on log page
Allow adding new workout sessions from the calendar view
Remove the --use-mirrors option for pip, this has been deprecated
Merge pull request #60 from rolandgeider/equipment

Allow users to evaluate their workout session