auth: Move `standard_relay_params` of SAMLAuthBackend to `SocialAuthMixin`.
auth: Move `standard_relay_params` of SAMLAuthBackend to `SocialAuthMixin`.

Earlier this `standard_relay_params` was used only for SAML auth,
now "Sign in with Apple" also requires this to store those params
in session for reuse. So, this acts as a prep commit for "Sign in
with Apple" auth support.

brainwane/zulip,hackerkid/zulip,shubhamdhama/zulip,kou/zulip,timabbott/zulip,eeshangarg/zulip,rht/zulip,timabbott/zulip,kou/zulip,kou/zulip,timabbott/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,synicalsyntax/zulip,eeshangarg/zulip,timabbott/zulip,timabbott/zulip,zulip/zulip,timabbott/zulip,showell/zulip,punchagan/zulip,synicalsyntax/zulip,eeshangarg/zulip,shubhamdhama/zulip,synicalsyntax/zulip,rht/zulip,synicalsyntax/zulip,punchagan/zulip,brainwane/zulip,brainwane/zulip,hackerkid/zulip,synicalsyntax/zulip,zulip/zulip,showell/zulip,showell/zulip,andersk/zulip,kou/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,showell/zulip,kou/zulip,zulip/zulip,punchagan/zulip,kou/zulip,shubhamdhama/zulip,synicalsyntax/zulip,rht/zulip,shubhamdhama/zulip,andersk/zulip,punchagan/zulip,rht/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,synicalsyntax/zulip,brainwane/zulip,eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,shubhamdhama/zulip,rht/zulip,shubhamdhama/zulip,timabbott/zulip,rht/zulip,showell/zulip,brainwane/zulip,shubhamdhama/zulip,andersk/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,andersk/zulip,zulip/zulip,andersk/zulip