(core) Add new UI for writing memos

Summary:
Adds a new UI for writing access rule memos.

Migrates old memos (written as Python comments) to the new UI.

Test Plan: Browser and migration tests.

Reviewers: jarek, dsagal

Reviewed By: jarek

Subscribers: dsagal, paulfitz

Differential Revision: https://phab.getgrist.com/D3726

static/locales/en.client.json

@@ -587,7 +587,8 @@
       "EnterCondition": "Enter Condition",
       "RemoveRulesMentioningTable": "Remove {{- tableId }} rules",
       "RemoveRulesMentioningColumn": "Remove column {{- colId }} from {{- tableId }} rules",
-      "RemoveUserAttribute": "Remove {{- name }} user attribute"
+      "RemoveUserAttribute": "Remove {{- name }} user attribute",
+      "MemoEditorPlaceholder": "Type a message..."
     },
     "PermissionsWidget": {
       "AllowAll": "Allow All",