Initially proposed by #2376 and fixes #3068 as well.

From https://godoc.org/crypto/tls#Config

    ServerName is used to verify the hostname on the returned
    certificates unless InsecureSkipVerify is given. It is also included
    in the client's handshake to support virtual hosting unless it is
    an IP address.

This is needed for certificate validation without InsecureSkipVerify.

This is useful especially to check whether we fetch right attributes, using
right LDAP search base and in right order.

This is feature is workaround for #2628 (JumpCloud) and some other services
that allow LDAP search only under BindDN user account, but not allow any LDAP
search query in logged user DN context.

Such approach is an alternative to minimal permissions security pattern for
BindDN user.

According to RFC 4511 4.2.1. Processing of the Bind Request "Clients may send
multiple Bind requests to change the authentication and/or security
associations or to complete a multi-stage Bind process. Authentication from
earlier binds is subsequently ignored."

Therefore we should not use 2 connections, but single one just sending two bind
requests.

This is fix-up for 573305f. Forgot to fetch AttributeUsername value from the
LDAP server, so the setting was effectively not working as intended.

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.

The IsAdmin flag is set based on whether the admin filter
returned any result. The admin filter is applied with the user dn
as the search root.

In the future, we should update IsAdmin as well on each login.
Alternately, we can have a periodic sync operation.

Removed trailing whitespaces with

```sh
for file in `grep -r " $" * | cut -d : -f 1 | sort | uniq | grep -v " "`; do sed -i 's/[ \t]*$//' $file; done
```

and some handwork.