feat: Implement dynamic subscription feature with enhanced personalisation

Introduced a fetch request within subscribeFormHandler.js allowing users to subscribe to the site’s newsletter without refreshing the page.
Added a context processor within the subscription app, enabling personalised greetings across the site. If a user has subscribed, they are greeted with a tailored message; otherwise, a subscription form is displayed. The tailored message persists across sessions due to being stored in the session data. Also, the application verifies that the email being used is not already associated with a logged-in account and displays the appropriate message
Made the subscription form accessible only to logged-in users. This allows authenticated users to set the frequency of their newsletters (e.g., weekly, monthly) or unsubscribe. Currently, the user interface for managing subscriptions is not yet available but will be added in a future update.
Removed obsolete JavaScript files that were previously used to manage the subscription form:
- Removed subscribeElements.js as its functionality is now handled by the backend.
- Removed jwtToken.js since a secure token implementation is now managed server-side.
Added signals to ensure that email is being stored in lowercase

Workflow:

Users log into their account, navigate to the homepage, scroll to the bottom of the page, and subscribe.
Upon successful subscription, users are greeted with a personalised message.
Refresh the page and the message is there
Logout and there is no subscription form email

To-do:

Notify the admin when a new subscription is made.
Develop a UI for users to manage their subscription preferences and unsubscribe if they desire

EgbieAndersonUku1 / Fullstack-fruit-and-veg