Support for imperial units

Currently, developemnt for imperial units was happening in the
feature/routine-generator branch, but is being moved out so it can
be finished independently.

See #70
Copy AbstractWeight from routine-generator branch
Use correct weight units for bmi and basal rate
Replace mention of kg in templates

The user's weight unit it now used instead
Add support for grams and ounces
Fix indentation
Add helper tag to translate weight units
Load wger_extras tags in dashboard template
Make user parameter for trans_weight_unit optional

In this case, metric units are used
AbstractWeights can be initialised with g and oz
Add convenience function to return units used
Use units in nutritional values calculators
Refactor some of the nutrition calculations
Output correct weight unit for plan in mobile
Translate weight units in gym user overview
Use property to check for use of metric units
All ingredients in English have ounces as unit

Ounces (or liquid ounces) are now an available option for all English ingredients.
Pass user to render_weight_log

Needed to correctly render the user's weight unit. Since tags have no access to
the global context we have to pass the variables explicitly.
Right align nutritional values for plan
Small improvements to modal headers in mobile view