hooks: New mechanism to deprecate hooks

I plan on splitting authFailure into authnFailure and authzFailure so
that separate authentication and authentication plugins can coexist
peacefully. This change will make it possible to mark the authFailure
hook as deprecated (which simply logs a warning).

src/static/js/pluginfw/hooks.js

@@ -1,10 +1,33 @@
 var _ = require("underscore");
 
+// Maps the name of a server-side hook to a string explaining the deprecation
+// (e.g., 'use the foo hook instead').
+//
+// If you want to deprecate the fooBar hook, do the following:
+//
+//     const hooks = require('ep_etherpad-lite/static/js/pluginfw/hooks');
+//     hooks.deprecationNotices.fooBar = 'use the newSpiffy hook instead';
+//
+exports.deprecationNotices = {};
+
+const deprecationWarned = {};
+
+function checkDeprecation(hook) {
+  const notice = exports.deprecationNotices[hook.hook_name];
+  if (notice == null) return;
+  if (deprecationWarned[hook.hook_fn_name]) return;
+  console.warn('%s hook used by the %s plugin (%s) is deprecated: %s',
+      hook.hook_name, hook.part.name, hook.hook_fn_name, notice);
+  deprecationWarned[hook.hook_fn_name] = true;
+}
+
 exports.bubbleExceptions = true
 
 var hookCallWrapper = function (hook, hook_name, args, cb) {
   if (cb === undefined) cb = function (x) { return x; };
 
+  checkDeprecation(hook);
+
   // Normalize output to list for both sync and async cases
   var normalize = function(x) {
     if (x === undefined) return [];