Assign exercises to body parts

[lortza]

I'd like to change the dropdowns on the new exercise log page to filter the exercise based on the body part.

• Set up new join table: body_part_exercises
• up the model relationships
  • A body part can have many exercises and an exercise can belong to many exercises
• do the data entry
• add body part checklists to the exercise form (see meal_plan and recipes example)
• add the JS to make the dropdowns filter