Added two new tables, exercise_user_pairs and workout_user pairs to keep track of who "owns" which exercises.

User 0 will be reserved, and will hold a bank of selectable exercises.

Revert "Removed user_id from exercise_set_log, workout_log tables.  Moved to exercise_table, workout_table, so each user can have"

This reverts commit cdbedfe5bcf6d2f0290306314a7c409e9772eccf.