FEATURE: add create event button to editor

[poka-IT]

Summary

This pull request introduces the following enhancements to the Discourse Calendar plugin:

1. Event Creation Button Placement: The event creation button is now positioned in the same location as the previous plugin, ensuring a consistent user experience.
2. Behavior Adjustment: The button now inserts the event text tag at the top of the message, regardless of the cursor position, improving usability.
3. Configurable Categories: The list of categories where the button appears is configurable. Appears everywhere if not set, and nowhere if set to -1.

Implementation Details

• The button's display logic has been refined to ensure it only appears in specified categories and on the first message of topics.
• A new configuration option allows moderators to specify which categories should display the button.
• Subcategories are automatically included if present.

Screenshots

These changes aim to improve the usability and configurability of the event creation button, making it more intuitive and flexible for different forum setups.

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I'm sure this need to be review and improve by Discourse team, because it's my first Discourse plugin change.
This is actualy in production in https://forum.monnaie-libre.fr.