Update reserved usernames list (#18438)

Adding additional usernames which are already routes, remove unused ones.
In future, avoid reserving names as much as possible, use `/-/` in path instead.

models/user/user.go

@@ -576,6 +576,7 @@ var (
 		"api",
 		"assets",
 		"attachments",
+		"avatar",
 		"avatars",
 		"captcha",
 		"commits",
@@ -584,10 +585,7 @@ var (
 		"explore",
 		"favicon.ico",
 		"ghost",
-		"help",
-		"install",
 		"issues",
-		"less",
 		"login",
 		"manifest.json",
 		"metrics",
@@ -595,15 +593,15 @@ var (
 		"new",
 		"notifications",
 		"org",
-		"plugins",
 		"pulls",
 		"raw",
 		"repo",
+		"repo-avatars",
 		"robots.txt",
 		"search",
 		"serviceworker.js",
-		"stars",
-		"template",
+		"ssh_info",
+		"swagger.v1.json",
 		"user",
 		"v2",
 	}