Kibana doesn't redirect to correct page after session expiration

[jportner]

Kibana version: master

Describe the bug: When a user gets logged out from session expiration, their path is preserved in the URL's next query parameter. After the user logs in again, they should be redirected back to the previous page. This is not working properly; the user is simply landing on the /app/kibana page instead.

Steps to reproduce:

1. Start Kibana with xpack.security.session.lifespan: 90000
2. Log in and navigate to any app (such as a Dashboard)
3. Wait for the session to expire
4. Log in again

Expected behavior: The user should be redirected to the page they were previously on.

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/kibana-security (Team:Security)

[jportner]

This is happening because the Security plugin code that creates the redirect URL (with next query param) is stripping the basePath out. Then when the login page's triggers parseNext, it fails to detect this as a valid path and redirects the user to the basePath as a failsafe.