feat: adding category implemented (#20)

client/src/app/app-routing.module.ts

client/src/app/app.component.html

client/src/app/app.module.ts

...p/categories/admin-category/admin-category.component.html

...p/categories/admin-category/admin-category.component.scss

client/src/app/categories/admin/admin.component.html

client/src/app/categories/admin/admin.component.scss

client/src/app/categories/admin/admin.component.spec.ts

client/src/app/categories/admin/admin.component.ts

...nt/src/app/posts/post-details/post-details.component.html

client/src/app/posts/post-details/post-details.component.ts

server/src/controllers/categoryController/index.ts

server/src/controllers/postController/index.ts

server/src/models/post.ts