Added support for multiple LDAP servers in a auth source (go-gitea#6898)

[Rob61]

Fixes Feature-Request #6898

Added support for multiple LDAP(S) servers in a auth source.
Servers are separated by space characters.

Before this pull request:
func dial gets a string containing the LDAP server and try to connect.

With this pull request:
func dial gets a string containing the LDAP server or multiple LDAP servers separated by space character.
It splits the string into a list of strings and try to connect to the first, but will try the second, if the first fails, etc.
Leading to the possibility to provide a backup LDAP servers if the main server is not reachable e.g. because of maintaince.

Best
~Rob

[silverwind]

Suggestions:

• Split on comma instead of space, it's more intuitive I think
• Any reason not to parallelize the connections? If for example a server fails with a network timeout, all authorizations would need to wait for that timeout (or forever if there's not timeout). I could not find what the default timeout value for that module is on https://godoc.org/gopkg.in/ldap.v3 and it doesn't look we set one either.
• Docs need to be updated probably (at least a hint in the UI)

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 months. Thank you for your contributions.

[lunny]

We needs some tests for this PR.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs during the next 2 months. Thank you for your contributions.

[lunny]

Please resolve the conflicts.

[wxiaoguang]

It has been stale for a long time and I can't think of a way to handling it other than closing it. Feel free to reopen if there's any new progress and I could also help.