feat: Add option to disable builtin authentication.

Setting ENABLE_INTERNAL_SIGNIN to false will disable the built-in
signin form, should the administrator prefer to limit users to SSO.

Continuation of forgejo/forgejo#6076

templates/user/auth/oauth_container.tmpl

@@ -1,7 +1,9 @@
 {{if or .OAuth2Providers .EnableOpenIDSignIn}}
+{{if .EnableInternalSignIn}}
 <div class="divider divider-text">
 	{{ctx.Locale.Tr "sign_in_or"}}
 </div>
+{{end}}
 <div id="oauth2-login-navigator" class="tw-py-1">
 	<div class="tw-flex tw-flex-col tw-justify-center">
 		<div id="oauth2-login-navigator-inner" class="tw-flex tw-flex-col tw-flex-wrap tw-items-center tw-gap-2">

templates/user/auth/signin_inner.tmpl

@@ -10,6 +10,7 @@
 		{{end}}
 	</h4>
 	<div class="ui attached segment">
+		{{if .EnableInternalSignIn}}
 		<form class="ui form" action="{{.SignInLink}}" method="post">
 			{{.CsrfTokenHtml}}
 			<div class="required field {{if and (.Err_UserName) (or (not .LinkAccountMode) (and .LinkAccountMode .LinkAccountModeSignIn))}}error{{end}}">
@@ -43,6 +44,7 @@
 				</button>
 			</div>
 		</form>
+		{{end}}
 
 		{{template "user/auth/oauth_container" .}}
 	</div>