Frontend refactor API

[onucsecu2]

Change the REST Api src/app/constants.ts

export const restAPI = {
  LOGIN: '/login/access-token',
  SIGN_UP: '/users/open',
  SNIPPETS: '/snippets',
  LANGUAGES: '/languages',
  TAGS: '/tags'
};

to

export const restAPI = {
  LOGIN: '/auth/login',
  SIGN_UP: '/auth/registration',
  SNIPPET: '/snippet',
  LANGUAGE: '/language',
  TAG: '/tag'
};

Re structure the input/output json format.