Use componentDidMount in place of componentWillMount where possible

This fixes a common React warning we see. Most of these components should be using constructors instead, however componentDidMount is just as good (and doesn't require converting most of these).

Conversion to classes will be done in a later stage of React warning fixes.

For https://github.com/vector-im/riot-web/issues/12877

src/components/structures/auth/ForgotPassword.js

@@ -69,7 +69,7 @@ export default createReactClass({
         };
     },
 
-    componentWillMount: function() {
+    componentDidMount: function() {
         this.reset = null;
         this._checkServerLiveliness(this.props.serverConfig);
     },

src/components/structures/auth/Login.js

@@ -113,7 +113,7 @@ export default createReactClass({
         };
     },
 
-    componentWillMount: function() {
+    componentDidMount: function() {
         this._unmounted = false;
 
         // map from login step type to a function which will render a control

src/components/structures/auth/PostRegistration.js

@@ -37,7 +37,7 @@ export default createReactClass({
         };
     },
 
-    componentWillMount: function() {
+    componentDidMount: function() {
         // There is some assymetry between ChangeDisplayName and ChangeAvatar,
         // as ChangeDisplayName will auto-get the name but ChangeAvatar expects
         // the URL to be passed to you (because it's also used for room avatars).

src/components/structures/auth/Registration.js

@@ -120,7 +120,7 @@ export default createReactClass({
         };
     },
 
-    componentWillMount: function() {
+    componentDidMount: function() {
         this._unmounted = false;
         this._replaceClient();
     },