Add auto focus to Delete DB modal

This ticket:
- Updates ModalView to default to focus on the first visible text field
by default.
- Updates the error message when the user fails to type in the database
name correctly.

Closes COUCHDB-2508
We usually use `this.$` and no `$(selector)` - I assume you use a global selector as you were the context of this was wrong?

```javascript
      this.$('.modal').on('shown', function () {
        this.$('input').focus();
      }.bind(this));
```