Add Failed or Missed reps to `exercise_event`

I often find myself wanting to simply record a count of failed or missed reps on an exercise event.

Add the following column to the field
```
exercise_event
missed_reps: integer
```
and updating forms accordingly

Locate the exercise_event schema or model and the forms used to create or edit exercise events. Confirm how existing integer fields are persisted and represented, then verify that missed_reps can be entered and saved wherever those forms apply.

Written by the indexing model from the issue text.