drop support for usernames

Mojang has disabled their legacy skins API:
https://twitter.com/MojangSupport/status/964511258601865216

With their API rate limits, it's now practially impossible
for us to support usernames.

Fixes #142. The default parameter allows using:

- UUID
- URL
- MHF_Alex
- MHF_Steve
- Alex
- Steve

Contrary to UUIDs, using alex/steve doesn't redirect
and instead provides the skin from a locally stored file.

lib/public/javascript/crafatar.js

@@ -1,4 +1,4 @@
-var valid_user_id = /^([0-9a-f-A-F-]{32,36}|[a-zA-Z0-9_]{1,16})$/; // uuid|username
+var valid_user_id = /^[0-9a-f-A-F-]{32,36}$/; // uuid
 var xhr = new XMLHttpRequest();
 
 xhr.onload = function() {
@@ -9,24 +9,14 @@ xhr.onload = function() {
     status[key] = elem[key];
   });
 
-  var textures = status["textures.minecraft.net"] !== "green";
-  var session = status["sessionserver.mojang.com"] !== "green";
-  var skins = status["skins.minecraft.net"] !== "green";
-  var error = null;
+  var textures_err = status["textures.minecraft.net"] !== "green";
+  var session_err = status["sessionserver.mojang.com"] !== "green";
 
-  if (textures || session && skins) {
-    error = "all";
-  } else if (skins) {
-    error = "username";
-  } else if (session) {
-    error = "UUID";
-  }
-
-  if (error) {
+  if (textures_err || session_err) {
     var warn = document.createElement("div");
     warn.setAttribute("class", "alert alert-warning");
     warn.setAttribute("role", "alert");
-    warn.innerHTML = "<h5>Mojang issues</h5> Mojang's servers are having trouble <i>right now</i>, this may affect <b>" + error + "</b> requests at Crafatar. <small><a href=\"https://help.mojang.com\" target=\"_blank\">check status</a>";
+    warn.innerHTML = "<h5>Mojang issues</h5> Mojang's servers are having trouble <i>right now</i>, this may affect requests at Crafatar. <small><a href=\"https://help.mojang.com\" target=\"_blank\">check status</a>";
     document.querySelector("#alerts").appendChild(warn);
   }
 };