Issue #7 🎫: category form page

[ITurres]

CaegoriesController

• Updated create action to override icon attr, store current user,
  use flash messages, and redirect to the categories index page.
• Updated update action to use flash messages and redirect to
  the categories index page.
• Update destroy action to use flash messages and redirect to
  the categories index page.
• Removed json and html responses from create, update and destroy actions.
• Removed show and edit actions.
• Created a random_icon_color method to generate a random colour for the category icon.
• Created set_category_icon to use placeholder icons.

Markup and Styles

• Updated the edit, new and _form views.
• Styles have been added following the design guidelines. The button's styles have been moved to a separate file buttons.css.

Tests

• Integration tests for the categories#new view.
• Unit tests for the categories controller actions.
• A login_helper file has been created to facilitate the user login for testing.

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