back button on all tour modals

bookwyrm/templates/guided_tour/book.html

@@ -139,6 +139,13 @@
                 on: "left",
             },
             buttons: [
+                {
+                    action() {
+                        return this.back();
+                    },
+                    secondary: true,
+                    text: "{% trans 'Back' %}",
+                },
                 {
                     action() {
                         return this.complete();

bookwyrm/templates/guided_tour/user_groups.html

@@ -107,6 +107,13 @@
             {% trans ' button to continue the tour.' %}",
             title: "{% trans 'Save your group' %}",
             buttons: [
+                {
+                    action() {
+                        return this.back();
+                    },
+                    secondary: true,
+                    text: "{% trans 'Back' %}",
+                },
                 {
                     action() {
                         return this.complete();

bookwyrm/templates/guided_tour/user_profile.html

@@ -127,6 +127,13 @@ tour.addSteps([
             on: "right",
         },
         buttons: [
+            {
+                action() {
+                    return this.back();
+                },
+                secondary: true,
+                text: "{% trans 'Back' %}",
+            },
             {
                 action() {
                     return this.complete();