Bug 31064: Wrap local login with stylable element

Bug 31064: Wrap local login with stylable element

Test plan:
0. Apply patch
1. Go to http://localhost:8081/cgi-bin/koha/admin/preferences.pl?op=search&searchfield=OPACUserCSS
2. Set syspref to the following:
.local-login {
      display: none;
}
3. Go to http://localhost:8080/cgi-bin/koha/opac-main.pl
3b. Note that the local login boxes are hidden
    ("Log in to your account:" remains so that other login options can
    be supplied by OpacUserJS)

4. Click on "Log in to your account" on the top toolbar
4b. Note that the local login boxes are hidden
    ("Log in to your account:" remains so that other login options can
    be supplied by OpacUserJS)

5. Go to http://localhost:8080/cgi-bin/koha/opac-user.pl
5b. Note that the local login boxes are hidden
    ("Log in to your account" remains so that other login options can be
    supplied by OpacUserJS)

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Tomás Cohen Arazi <tomascohen@theke.io>