Bug 26695: Add modal "shown" event to set cursor focus

Bug 26695: Add modal "shown" event to set cursor focus

This patch attaches a focus() call to the login modal's "shown.bs.modal"
event so that the cursor focus is set to the username field when the
login modal is displayed.

To test, apply the patch and confirm that your OPAC uses the regular
login process--not casAuthentication or GoogleOpenIDConnect.

Click the "Log in to your account" link in the header. A the login form
should appear in a modal dialog, and the focus should be on the username
field.

The same should be true if you trigger the modal from other
locations, for instance: "Log in to add tags" on the bibliographic
detail page or the tag cloud page.

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>