Let MinUnitAccessMode return correct perm (go-gitea#18675)

- Don't let `TypeExternalTracker` or `TypeExternalWiki` influence the
minimal permission, as they won't be higher than read. So even if all
the other ones are write, these 2 will ensure that's not higher than
read.
- Partially resolves go-gitea#18572 (Point 1,2,5?)

Co-authored-by: zeripath <art27@cantab.net>

models/unit/unit.go

@@ -328,7 +328,12 @@ func AllUnitKeyNames() []string {
 // MinUnitAccessMode returns the minial permission of the permission map
 func MinUnitAccessMode(unitsMap map[Type]perm.AccessMode) perm.AccessMode {
 	res := perm.AccessModeNone
-	for _, mode := range unitsMap {
+	for t, mode := range unitsMap {
+		// Don't allow `TypeExternal{Tracker,Wiki}` to influence this as they can only be set to READ perms.
+		if t == TypeExternalTracker || t == TypeExternalWiki {
+			continue
+		}
+
 		// get the minial permission great than AccessModeNone except all are AccessModeNone
 		if mode > perm.AccessModeNone && (res == perm.AccessModeNone || mode < res) {
 			res = mode