Add account migration UI (#11846)

Fix #10736

- Change data export to be available for non-functional accounts
- Change non-functional accounts to include redirecting accounts

spec/fabricators/account_migration_fabricator.rb

@@ -0,0 +1,6 @@
+Fabricator(:account_migration) do
+  account
+  target_account
+  followers_count 1234
+  acct 'test@example.com'
+end