Add an indicator in the conversation list for unread reactions

[michaelstambach]

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• I have read how to contribute to this project
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Contributor checklist

• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• Virtual "sdk_gphone64_x86_64", Android 16 (Android Studio default)

• My contribution is fully baked and ready to be merged as is
• I ensure that all the open issues my contribution fixes are mentioned in the commit message of my first commit using the Fixes #1234 syntax

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Description

This commit adds an indicator to the conversation list for conversations with unread reactions. Currently, if a reaction to message is received a notification is sent but inside the app there is no way to see from which chat this notification arrived. This PR changes this so that a small heart is displayed next to the unread indicator if there was a reaction to an outgoing message.
The indicator is not shown if there are also unread mentions to save space.

I tried to implement this similarly to how the unread mentions indicator is implemented. This required adding a new column to the thread table to track the reactions to the users own messages. This is my first time contributing to this project so I hope I have done everything as intended. I tested my changes and they seem to work as intended.

See the image below for an example of the indicator showing reactions on chats.