WIP: Add rudimentary LDAP group support

[stertingen]

Targeting #2212.

Features:

• Restrict automatic registration with a group filter
• Allow admin access based on a group filter
• Arbitrary kinds of LDAP groups are supported (custom filter with custom user attribute)

Sub-tasks

• Add configuration options for the admin backend
• Extend login code with new group options
• Tests for new options suitable for the test OpenLDAP server
• Test with another LDAP setup (not using dn as user attribute in group)
• Refactor LDAP code to remove duplication (esp. SearchEntry and SearchEntries)
• Add CLI options (see Add CLI commands to manage LDAP authentication source #6681)
• Test CLI options

[stertingen]

Well, I forgot to apply the changes to SearchEntries().

[codecov-io]

Codecov Report

Merging #7349 into master will increase coverage by 0.16%.
The diff coverage is 56.38%.

@@            Coverage Diff             @@
##           master    #7349      +/-   ##
==========================================
+ Coverage    41.4%   41.56%   +0.16%     
==========================================
  Files         478      478              
  Lines       63944    64028      +84     
==========================================
+ Hits        26473    26616     +143     
+ Misses      34021    33954      -67     
- Partials     3450     3458       +8

Impacted Files	Coverage Δ
modules/auth/auth_form.go	100% <ø> (ø)	⬆️
routers/admin/auths.go	33.6% <100%> (+1.37%)	⬆️
modules/auth/ldap/ldap.go	55% <53.93%> (+0.59%)	⬆️
models/repo.go	49.02% <0%> (+0.38%)	⬆️
models/login_source.go	27.08% <0%> (+0.78%)	⬆️
routers/repo/view.go	43.54% <0%> (+1.01%)	⬆️
models/repo_list.go	73.09% <0%> (+1.01%)	⬆️
modules/context/auth.go	24.71% <0%> (+4.49%)	⬆️
models/unit.go	67.56% <0%> (+5.4%)	⬆️
... and 2 more

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[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]

CI is still failed and will you finish the left sub tasks on the comment?