Always allow session owners to join own sessions + only list active trackers in WebUI

[xacrimon]

Fixes #13595

This PR makes a couple of changes:

• Filters out trackers marked as terminated in the WebAPI, preventing them from being shown in the session view.
• Changes the RBAC engine to always allow session owners to join their own session.
• Make web use the -teleport-internal-join principal when joining sessions.
• Readd some changes we lost in a refactor to allow users to join with the special principal.

[Joerger]

I'm seeing a few remaining issues here:

• tsh join fails from the cli with ERROR: access denied to -teleport-internal-join connecting to 33e9a6a4-5986-40f7-b07c-90e6340c38c4 on cluster example.com
• listed sessions in the webui have the host use listed 3 times, like: admin [], admin [], admin []
  • created follow up PR: Remove duplicate parties in kube and ssh session trackers. #13753
• Zombie sessions: If you close the server while sessions are open, then restart, you'll see the sessions are still in the webui. They are also still joinable somehow, and never go through the linger and die logic, because they think they still have an active participant.
  • Opened a separate issue for this one - Terminated session viewable and joinable in webui after restarting Node #13714

Not sure if these are directly related to this issue or should be fixed in follow up PRs

[xacrimon]

Checked and my PR should now address nr1.

[github-actions]

@xacrimon See the table below for backport results.

Branch	Result
branch/v10	Create PR