Exercise selection in workout form should be searchable

[diegocasmo]

The current <WorkoutForm/> simply fetches all exercises and renders them as options to select from. This is not ideal, since the number of exercises a user has created might be too many, which means the form will end up having a bad UX.

The goal of this task is to make the exercises drop-down select box searchable. It should render ~10 exercises initially, but it should allow the user to type the name of the exercise they want to add to the workout, and the drop-down should show the updated options according to the query.

Consider using something like the react-select for this task.