Avoid race condition in LoginPage component

[reiallenramos]

Closes #48

Changes:

• set a flag to decide whether to display the Login snackbar

Screenshots:
When refreshing the User page:

• if the reroute from /login to /user finishes sooner than the request to get LOGIN_MESSAGE resolves, the snackbar is never shown.
  

[liangyuanruo]

As stated here, #48 (comment), the implementation is injected server side because we prefer to be able to update the displayed message without code changes.

That said, we are happy to accept a PR that resolves the race condition brought up in the issue.

[reiallenramos]

@liangyuanruo makes sense. I'm just trying to wrap my head around your comment. Does this mean if you change an environment variable in production, you wont have to re-deploy to reflect the changes? Express picks it up instantly? 🤔

[liangyuanruo]

@liangyuanruo makes sense. I'm just trying to wrap my head around your comment. Does this mean if you change an environment variable in production, you wont have to re-deploy to reflect the changes? Express picks it up instantly? 🤔

No, it means that changing the message is considered a system configuration, which does not require a code commit.

[liangyuanruo]

Thanks, code looks good to me. Final thing - could you switch the pull request target to develop? That is our development mainline, and we will perform a merge into master for our next release cycle.