Refactor internal routers (partial backport, auth token const time comparing) (#32473) (#32479)

Partially backport #32473. LFS related changes are not in 1.22, so skip
them.

1. Ignore non-existing repos during migrations
2. Improve ReadBatchLine's comment
3. Use `X-Gitea-Internal-Auth` header for internal API calls and make
the comparing constant time (it wasn't a serous problem because in a
real world it's nearly impossible to timing-attack the token, but indeed
security related and good to fix and backport)
4. Fix route mock nil check

modules/private/internal.go

@@ -43,7 +43,7 @@ Ensure you are running in the correct environment or set the correct configurati
 	req := httplib.NewRequest(url, method).
 		SetContext(ctx).
 		Header("X-Real-IP", getClientIP()).
-		Header("Authorization", fmt.Sprintf("Bearer %s", setting.InternalToken)).
+		Header("X-Gitea-Internal-Auth", fmt.Sprintf("Bearer %s", setting.InternalToken)).
 		SetTLSClientConfig(&tls.Config{
 			InsecureSkipVerify: true,
 			ServerName:         setting.Domain,