feat(RM-40): Option to not send panels to a channel

[DanPlayz0]

🚨 This pull request requires that TicketsBot-cloud/database#2 is merged.

This pull request introduces changes to handle cases where ChannelId and MessageId can be nil, improving robustness and error handling across the panel management backend and frontend. The most significant updates include making these fields nullable, adjusting logic to handle nil values gracefully, and updating the frontend to reflect these changes.

Backend Changes:

• Made ChannelId and MessageId fields nullable in the panelBody and panelMessageData structs, and updated methods like IntoPanelMessageData and panelIntoMessageData to handle nil values safely. [1] [2]
• Updated panel creation, deletion, and update logic to account for nil ChannelId or MessageId, ensuring no operations are attempted on invalid or missing data. [1] [2] [3]
• Enhanced validation logic to skip validation for nil ChannelId and ensure proper handling of nullable fields.

Frontend Changes:

• Updated ChannelDropdown in the panel creation form to allow a null value, with a "None" label to indicate no channel is selected.
• Modified the panel management view to display "None" for panels without a channel and disable the "Resend" button if channel_id is null.

Notes

This PR was originally created in May, and was later updated; original changes can be found by comparing commits 482902c9163f3cc2dfb6ce587e55561ce9a7ea27 & 20a65e6288a20667e5581e49a62d43df64d0298e