client/web: move auth session creation out of /api/auth

Splits auth session creation into two new endpoints:

/api/auth/session/new - to request a new auth session

/api/auth/session/wait - to block until user has completed auth url

Updates tailscale/corp#14335

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f