Photomap add: UI does not enforce caption requirement, so API upload then fails

At present the app allows the caption to be left blank.

The photo submission then fails with this error message, with the API rejecting the submission:

```
There was a problem uploading your photo:
Not all data about the location was sent - the caption field was missing.
```

The UI should be updated to make sure the caption field is not optional to avoid the user getting as far as this.