Greatly simplify history management code (#2230)

Fixes #2220

This drops the ability to shift+click on “Back” to get back to a pinned
column, but that was inconsistent, broken, and undocumented.

This also brings us slightly closer to upstream.

app/javascript/flavours/glitch/components/column_back_button.jsx

@@ -14,17 +14,15 @@ export default class ColumnBackButton extends React.PureComponent {
     multiColumn: PropTypes.bool,
   };
 
-  handleClick = (event) => {
-    // if history is exhausted, or we would leave mastodon, just go to root.
-    if (window.history.state) {
-      const state = this.context.router.history.location.state;
-      if (event.shiftKey && state && state.mastodonBackSteps) {
-        this.context.router.history.go(-state.mastodonBackSteps);
-      } else {
-        this.context.router.history.goBack();
-      }
+  handleClick = () => {
+    const { router } = this.context;
+
+    // Check if there is a previous page in the app to go back to per https://stackoverflow.com/a/70532858/9703201
+    // When upgrading to V6, check `location.key !== 'default'` instead per https://github.com/remix-run/history/blob/main/docs/api-reference.md#location
+    if (router.route.location.key) {
+      router.history.goBack();
     } else {
-      this.context.router.history.push('/');
+      router.history.push('/');
     }
   };