MM-64718 Improve validation of thread follower imports (#33287)

* MM-64718 Improve validation of thread follower imports

* Add additional test cases and restucture tests

server/i18n/en.json

@@ -5762,6 +5762,10 @@
     "id": "app.import.validate_direct_post_import_data.message_missing.error",
     "translation": "Missing required direct post property: message"
   },
+  {
+    "id": "app.import.validate_direct_post_import_data.thread_follower.error",
+    "translation": "Failed to validate direct post thread follower data."
+  },
   {
     "id": "app.import.validate_direct_post_import_data.unknown_flagger.error",
     "translation": "Direct post can only be flagged by members of the channel it is in. \"{{.Username}}\" is not a member."
@@ -5818,6 +5822,10 @@
     "id": "app.import.validate_post_import_data.team_missing.error",
     "translation": "Missing required Post property: Team."
   },
+  {
+    "id": "app.import.validate_post_import_data.thread_follower.error",
+    "translation": "Failed to validate post thread follower data."
+  },
   {
     "id": "app.import.validate_post_import_data.user_missing.error",
     "translation": "Missing required Post property: User."
@@ -5950,6 +5958,14 @@
     "id": "app.import.validate_team_import_data.type_missing.error",
     "translation": "Missing required team property: type."
   },
+  {
+    "id": "app.import.validate_thread_follower_data.empty.error",
+    "translation": "Import follower data empty."
+  },
+  {
+    "id": "app.import.validate_thread_follower_data.user_missing.error",
+    "translation": "Missing required follower property: user."
+  },
   {
     "id": "app.import.validate_user_channels_import_data.channel_name_missing.error",
     "translation": "Channel name missing from User's Channel Membership."