SAML Auth Provider - #1576

Ok, the login should now work, including automatic registration. The SAML provider is implemented in a very similar way to a Socialite plugin. It may be possible to refactor this into an actual socialite plugin. When I checked last week, such a plugin was not available. The configuration now allows for variables like

SAML_USER_NAME_ATTRIBUTE=username
SAML_DISPLAY_NAME_ATTRIBUTE=firstName|lastName
SAML_AUTO_REGISTER=true

I'll address the codeclimate issues in some upcoming commit.

Synchronizing groups now works the same as with the LDAP service. There is now an ExternalAuthService in BookStack\Auth\Access, that contains the common functionality. Ideally, someone would turn this into a proper socialite provider, but that needs to be maintained.

Thank you very much @Xiphoseer for your work here. Code looks great from a quick scan but I'll have a deeper review at some point soon. I'll probably focus on getting v0.27 done before then coming back to this, Hopefully so this is part of v0.28.

Don't worry too much about codeclimate issues btw, I'm always creating new codeclimate issues 😅

Is there an updated timeline for v0.28.0?
Definitely interested in some of the new features.

@antimatteraccord Nope, not really. It's in progress and will be done when it's ready.

Continued in #1787

As with #1787, This has now been merged ready for the next feature release 🎉

Again, Thank you very much @Xiphoseer.