Don't update last message of conversation with invisible message

[nickvergessen]

Scenario: Add reaction

1. Create a conversation
2. Write a chat message "Message 1"
3. Add reaction ❤️
4. Navigate to main page of talk
5. Assert last message is "You: Message 1" ✅
6. Open the conversation
7. Assert last message is "You: Message 1" ❌ see "❤️" instead

Scenario: Remove reaction

1. Create a conversation
2. Write a chat message "Message 1"
3. Add reaction ❤️
4. Remove reaction ❤️
5. Navigate to main page of talk
6. Assert last message is "You: Message 1" ❌ see "You deleted a reaction" instead
7. Open the conversation
8. Assert last message is "You: Message 1" ❌ see "You deleted a reaction" instead

Scenario: Remove message

1. Create a conversation
2. Write a chat message "Message 1"
3. Write a chat message "Message 2"
4. Delete "Message 1"
5. Navigate to main page of talk
6. Assert last message is "You: Message 2" ❌ see "You deleted a message" instead
7. Open the conversation
8. Assert last message is "You: Message 2" ❌ see "You deleted a message" instead