JIRA-002 Implemented Book reordering

[AnjaBasara]

What?

Added logic for reordering books on the index page, as well as logic for reordering edited book after the stock_amount has been changed. For more background, see ticket #JIRA-002.

Why?

These changes complete the user book management experience, ensuring that the books are displayed in the desired order.

How?

This includes a migration, updated factory and seeder, form request classes, updated controller, new repository and a service for book management. Used form request classes to move validation out of controllers and moved business logic to a service.

Testing?

Testing reordering books:

1. Open index page
2. Choose a book with a positive stock amount
3. Move the book up or down by entering a number in the appropriate field
4. Click Move button
5. The book is appropriately reordered

Testing changing book stock amount:

1. Open index page
2. Choose a book and click Edit button
3. Change stock amount (if positive, set it to zero, or if zero set it to a positive number)
4. Click Update button
5. The book is appropriately reordered

Screenshots?

Anything else?

It would be good to consider having one set of Up, Down and Move (with book selection) since this way the user can enter multiple "ups" and "downs" but the Move button is tied to one book, so one set of those fields feels more intuitive.