Refactor LFS SSH and internal routers (#32473)

Gitea instance keeps reporting a lot of errors like "LFS SSH transfer connection denied, pure SSH protocol is disabled". When starting debugging the problem, there are more problems found. Try to address most of them:

* avoid unnecessary server side error logs (change `fail()` to not log them)
* figure out the broken tests/user2/lfs.git (added comments)
* avoid `migratePushMirrors` failure when a repository doesn't exist (ignore them)
* avoid "Authorization" (internal&lfs) header conflicts, remove the tricky "swapAuth" and use "X-Gitea-Internal-Auth"
* make internal token comparing constant time (it wasn't a serous problem because in a real world it's nearly impossible to timing-attack the token, but good to fix and backport)
* avoid duplicate routers (introduce AddOwnerRepoGitLFSRoutes)
* avoid "internal (private)" routes using session/web context (they should use private context)
* fix incorrect "path" usages (use "filepath")
* fix incorrect mocked route point handling (need to check func nil correctly)
* split some tests from "git general tests" to "git misc tests" (to keep "git_general_test.go" simple)

Still no correct result for Git LFS SSH tests. So the code is kept there
(`tests/integration/git_lfs_ssh_test.go`) and a FIXME explains the details.

modules/lfstransfer/backend/util.go

@@ -20,11 +20,11 @@ import (
 
 // HTTP headers
 const (
-	headerAccept        = "Accept"
-	headerAuthorisation = "Authorization"
-	headerAuthX         = "X-Auth"
-	headerContentType   = "Content-Type"
-	headerContentLength = "Content-Length"
+	headerAccept            = "Accept"
+	headerAuthorization     = "Authorization"
+	headerGiteaInternalAuth = "X-Gitea-Internal-Auth"
+	headerContentType       = "Content-Type"
+	headerContentLength     = "Content-Length"
 )
 
 // MIME types