Store redirect URL in a session variable

[zbynek]

Fixes https://issues.jenkins.io/browse/SECURITY-1833 from security advisory

Based on GitHub Oauth's solution https://github.com/jenkinsci/github-oauth-plugin/blob/master/src/main/java/org/jenkinsci/plugins/GithubSecurityRealm.java and @Wadeck 's suggestion in #17 (comment)

CC @jtnord @daniel-beck

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• [?] Ensure you have provided tests - that demonstrates feature works or fixes the issue

I didn't add any tests because I couldn't find any tests in thi repo. Setting them up is non-trivial because a mock implementation of GitLab is needed.

[zbynek]

@daniel-beck @Wadeck @jtnord weekly ping 🥺

[jtnord]

code change seems reasonable and to fix the issue on a quick glance.

[basil]

Released in 1.14.