Accessibility: Explicit <label>s

To follow Core's lead, we should update form control `` to have an explicit label target.

See https://make.wordpress.org/core/2019/10/14/improvements-in-media-component-accessibility-in-wordpress-5-3/

Example from the p2 post:
Old method:
```

My input

```

The label is implicit, but we can be more explicit to help accessibility overall:

```
My input

```

No files or tests are named. Start by locating the form-control markup throughout the Jetpack repository and compare it with the linked WordPress Core guidance; done means each applicable label has a matching for attribute and control id without breaking the existing forms.

Written by the indexing model from the issue text.