lib/config: Move the bcrypt password hashing to GUIConfiguration (#8028)

What hash is used to store the password should ideally be an
implementation detail, so that every user of the GUIConfiguration
object automatically agrees on how to handle it.  That is currently
distribututed over the confighandler.go and api_auth.go files, plus
tests.

Add the SetHasedPassword() / CompareHashedPassword() API to keep the
hashing method encapsulated.  Add a separate test for it and adjust
other users and tests.  Remove all deprecated imports of the bcrypt
package.

lib/api/api_auth_test.go

@@ -9,19 +9,20 @@ package api
 import (
 	"testing"
 
-	"golang.org/x/crypto/bcrypt"
+	"github.com/syncthing/syncthing/lib/config"
 )
 
-var passwordHashBytes []byte
+var guiCfg config.GUIConfiguration
 
 func init() {
-	passwordHashBytes, _ = bcrypt.GenerateFromPassword([]byte("pass"), 0)
+	guiCfg.User = "user"
+	guiCfg.HashAndSetPassword("pass")
 }
 
 func TestStaticAuthOK(t *testing.T) {
 	t.Parallel()
 
-	ok := authStatic("user", "pass", "user", string(passwordHashBytes))
+	ok := authStatic("user", "pass", guiCfg)
 	if !ok {
 		t.Fatalf("should pass auth")
 	}
@@ -30,7 +31,7 @@ func TestStaticAuthOK(t *testing.T) {
 func TestSimpleAuthUsernameFail(t *testing.T) {
 	t.Parallel()
 
-	ok := authStatic("userWRONG", "pass", "user", string(passwordHashBytes))
+	ok := authStatic("userWRONG", "pass", guiCfg)
 	if ok {
 		t.Fatalf("should fail auth")
 	}
@@ -39,7 +40,7 @@ func TestSimpleAuthUsernameFail(t *testing.T) {
 func TestStaticAuthPasswordFail(t *testing.T) {
 	t.Parallel()
 
-	ok := authStatic("user", "passWRONG", "user", string(passwordHashBytes))
+	ok := authStatic("user", "passWRONG", guiCfg)
 	if ok {
 		t.Fatalf("should fail auth")
 	}