Refactor: Move login out of models

[zeripath]

models does far too much. In particular it handles all UserSignin.

It shouldn't be responsible for calling LDAP, SMTP or PAM for signing in.

Therefore we should move this code out of models.

This code has to depend on models - therefore it belongs in services.

There is a package in services called auth and clearly this functionality belongs in there.

Plan:

• Change auth.Auth to auth.Method - as they represent methods of authentication.
• Move models.UserSignIn into auth
• Move models.ExternalUserLogin
• Move most of the LoginVia* methods to auth or subpackages
• Move Resynchronize functionality to auth
  • Involved some restructuring of models/ssh_key.go to reduce the size of this massive file and simplify its files.
• Move the rest of the LDAP functionality in to the ldap subpackage
• Re-factor the login sources to express an interfaces auth.Source?
  • I've done this through some smaller interfaces Authenticator and Synchronizable - which would allow us to extend things in future
• Now LDAP is out of models - need to think about modules/auth/ldap and I think all of that functionality might just be moveable
• Similarly a lot Oauth2 functionality need not be in models too and should be moved to services/auth/source/oauth2
  • modules/auth/oauth2/oauth2.go uses xorm... This is naughty - probably need to move this into models.
  • models/oauth2.go - mostly should be in modules/auth/oauth2 or services/auth/source/oauth2
• More simplifications of login_source.go may need to be done
• Allow wiring in of notify registration - this can now easily be done - but I think we should do it in another PR - see [Question] Send registration email when users login using LDAP for the first time #16178
• More refactors...?
  • OpenID should probably become an auth Method but I think that can be left for another PR
  • Methods should also probably be cleaned up - again another PR I think.
  • SSPI still needs more refactors.

[codecov-commenter]

Codecov Report

Merging #16199 (11c5a8a) into main (81091c4) will decrease coverage by 0.01%.
The diff coverage is 44.19%.

@@            Coverage Diff             @@
##             main   #16199      +/-   ##
==========================================
- Coverage   45.49%   45.48%   -0.02%     
==========================================
  Files         718      746      +28     
  Lines       84317    84315       -2     
==========================================
- Hits        38363    38348      -15     
- Misses      39791    39798       +7     
- Partials     6163     6169       +6     

Impacted Files	Coverage Δ
cmd/admin.go	0.00% <0.00%> (ø)
models/oauth2.go	27.27% <0.00%> (-0.73%)	⬇️
models/oauth2_application.go	69.11% <ø> (-0.29%)	⬇️
modules/cron/tasks_basic.go	85.43% <0.00%> (ø)
routers/init.go	57.14% <0.00%> (ø)
routers/web/user/auth_openid.go	0.00% <0.00%> (ø)
routers/web/user/setting/account.go	25.00% <0.00%> (ø)
routers/web/user/setting/security.go	32.78% <0.00%> (ø)
services/auth/oauth2.go	49.18% <0.00%> (-0.06%)	⬇️
services/auth/reverseproxy.go	0.00% <ø> (ø)
... and 81 more

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[zeripath]

updated to include #16465

[zeripath]

make lgtm work