LP1920273: improve Bootstrap self registration display

LP1920273: improve Bootstrap self registration display

This patch makes the form in the self-registration screen in the Bootstrap
opac a little responsive on smaller devices.

To test.

1. Turn on self-registration in the library settings.
2. Go to the self-registration screen and make the screen as small as possible.
3. Apply the patch.
4. Notice that the form labels and input boxes are now more aesthetic.
5. Create some self-registration requests to make sure the form captures the data.

Signed-off-by: Garry Collum <gcollum@gmail.com>
Signed-off-by: Rogan Hamby <rogan.hamby@gmail.com>
Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>