Consider following LDAP search query example:

    (&(objectClass=Person)(|(uid=%s)(mail=%s)))

Right now on first login attempt Gogs will use the text supplied on login form
as the newly created user name. In example query above the text matches against
both e-mail or user name. So if user puts the e-mail then the new Gogs user
name will be e-mail which may be undesired.

Using optional user name attribute setting we can explicitly say we want Gogs
user name to be certain LDAP attribute eg. `uid`, so even user will use e-mail
to login 1st time, the new account will receive correct user name.

This is follow-up for 56c66ee486 allowing admin
to see private repositories, even when not being member of them.

- copy link button: #1396, #1168, #1668,
- synxtax highlight: #1712, #1549, #1315, #670
- z-index: #1942