Allow configuration of substring search for searching sharees

This introduces new settings for the users and groups services.
"group_substring_filter_type" for the group services and
"user_substring_filter_type" for the users service. They allow to set
the type of LDAP filter that is used for substring user searches.
Possible values are: "initial", "final" and "any" to do either prefix,
suffix or full substring searches.

Fixes #547
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f