Tone of voice configuration for our AI assistant

Admins can now adjust the instructions given to our AI assistant. There are two settings:
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- Tone of voice: This controls the AI's overall tone when creating user-facing content, such as campaign material or share messages. For example, you can set it to be playful and casual.
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- Share messages: These are instructions for how the AI should generate share messages. For instance, you can instruct it to use inclusive language. These settings replace the previous "Must include at least one hashtag" toggle and are located on the "AI" section of the general "Settings" page in the backend.