Refactor rake notifications to use Notification model

[CloCkWeRX]

This PR refactors the planting and harvest reminder notifications in growstuff.rake to use the Notification model instead of calling NotifierMailer.deliver_later directly.

Key changes:

• Logic for identifying relevant plantings/harvests and generating notification content has been moved to a new ReminderService.
• Reminders now create Notification records, ensuring they appear in users' on-site inboxes.
• The Notification model triggers an email automatically via its after_create callback if the user has opted in.
• A NotificationsHelper was added with a reply_link implementation to handle linking from emails to conversations or posts.
• The Notification model now allows an optional notifiable association, facilitating aggregate reminders that don't point to a single object.
• New tests ensure that the rake tasks correctly delegate to the service and that the service creates notifications on the appropriate days.

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PR created automatically by Jules for task 3048820086930986872 started by @CloCkWeRX

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