Rewrite authentication logic with the API store

[carlobeltrame]

Fixes #355, fixes #318, fixes part of #359, fixes #364

• Cleans up all authentication logic and rewrites it using the API store in a much more readable way
• Adds a new API store function href that can be used to get the URI of a related entity without fetching it first
• Renames loaded Promises to load
• Fixes several bugs to do with reloading, posting or patching objects and the returned load Promises (and write tests for them)
• Renames the LoginController and all its related uses to AuthController and similar
• self links returned from a storeValueProxy (api.get() etc.) are now all absolute URIs (including the host), but in the store we still keep them as normalized, relative URIs, for easier debugging
• Adds fancy spinners to the login and logout buttons ✨

The main change is the simpler authentication logic. When before the logic was this:

  async function login (username, password) {
    return axios.post(process.env.VUE_APP_ROOT_API + '/login/login', { username: username, password: password })
      .then(resp => {
        if (resp.data.user !== 'guest') {
          this.loginSuccess()
          return true
        } else {
          return false
        }
      })
  },

now the equivalent code is written like this:

async function login (username, password) {
  const url = await href(get().auth(), 'login')
  return post(url, { username: username, password: password }).then(() => refreshLoginStatus())
}