Clean the utils and helpers folders

1. Remove helpers/immutable.js: The ideal scenario would be to work always with immutable objects, so let's get rid of this helper :)
2. Identify more improvements on those folders, check if code it's not being used, etc.

Move the corresponding components to a Page folder

Move the "pages" components to the Page folder. Naming convention: end file names with 'Page' postfix, eg. UsersPage.

Duck convention for naming action constants

Update to upperCase the last part of the constant
So instead of: export const GET_USER_ORG_USER_FETCHING = 'kaigi/all_users/get_user_org_user_fetching'
Do: export const GET_USER_ORG_USER_FETCHING = 'kaigi/all_users/GET_USER_ORG_USER_FETCHING'

Correct all linting issues

Fixing propTypes as well

Fix tests

Rename css folder to styles