Support the update of mint URL if it had to be changed by mint operator

package.json

@@ -1,6 +1,6 @@
 {
   "name": "minibits_wallet",
-  "version": "0.1.8-beta.31",
+  "version": "0.1.8-beta.32",
   "private": true,
   "scripts": {
     "android:clean": "cd android && ./gradlew clean",

src/models/Mint.ts

@@ -53,7 +53,7 @@ export type MintProofsCounter = {
  */
 export const MintModel = types
     .model('Mint', {
-        id: types.optional(types.string, () => generateId(8)),
+        id: types.optional(types.identifier, () => generateId(8)),
         mintUrl: types.string,
         hostname: types.maybe(types.string),
         shortname: types.maybe(types.string),