Rename gitea to forge in cli args and env variables (#339)

This PR renames `gitea` in cli args to `forge` and `GITEA` in environment variables to `FORGE` and adds the gitea names as aliases for the forge names.

Also closes #311

Reviewed-on: https://codeberg.org/Codeberg/pages-server/pulls/339

server/handler/handler_test.go

@@ -13,7 +13,7 @@ import (
 )
 
 func TestHandlerPerformance(t *testing.T) {
-	cfg := config.GiteaConfig{
+	cfg := config.ForgeConfig{
 		Root:           "https://codeberg.org",
 		Token:          "",
 		LFSEnabled:     false,