allow use of custom derivation path, and add way to increment derivation for existing

src/views/SeedBackupView.vue

@@ -28,6 +28,14 @@
         <i>Don't take a screenshot or send it to any online service.</i>
       </p>
 
+      <p v-if="numAccounts > 1">
+        <b class="text-orange-600">Note:</b> You have more than one identity
+        stored in this browser. If they are all based on the same seed as the
+        current identity, this one backup is sufficient; however, if you have
+        different seeds for other identities, you will have to back them up
+        separately.
+      </p>
+
       <div class="bg-slate-100 rounded-md overflow-hidden p-4 mb-4">
         <button
           class="block w-full text-center text-md uppercase bg-slate-500 text-white px-1.5 py-2 rounded-md"
@@ -60,6 +68,7 @@ import QuickNav from "@/components/QuickNav";
 @Component({ components: { AlertMessage, QuickNav } })
 export default class SeedBackupView extends Vue {
   activeAccount = null;
+  numAccounts = 0;
   showSeed = false;
   alertMessage = "";
   alertTitle = "";
@@ -73,6 +82,7 @@ export default class SeedBackupView extends Vue {
 
       await accountsDB.open();
       const accounts = await accountsDB.accounts.toArray();
+      this.numAccounts = accounts.length;
       this.activeAccount = R.find((acc) => acc.did === activeDid, accounts);
     } catch (err) {
       console.error("Got an error loading an identity:", err);