Issue #8 🎫: transaction form page

[ITurres]

TransactionDetailsController

• Updated before_action 'set_transaction_detail' to destroy only and except create.
• Filled show action to show transaction_detail.
• Declared categories and assign all categories to it at 'new' action.
• Updated create action to create new category and transaction association.
• Updated destroy action to use flash messages.
• Updated transaction_detail_params to permit category_id instead of
  user_id.
• Removed index, show, edit and update actions.

Schema and Migrations

• Created a new migration file to add :category_id column.
• Re-migrated schema.rb to reflect changes.

Markup and Styles

• Updated the new and _form views to match the wireframe style.
• Deleted the index and _category views.
• Gave styles to select elements.

Specs

• Unit specs for controller actions.
• Integration specs for the new view.

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