PRO-5666: fix /login issues

[boutell]

Fix the following issues with the /login URL:

• When you first arrived in an incognito window, the first login attempt didn't stick at all, but the second worked. This was due to a failure to cope with multiple Set-Cookie headers in the proxy logic. This was resolved using node-fetch and its support for access to all headers.
• When you attempted to visit /login while already logged in, you received an error and/or an undesirable page rendered directly by Apostrophe. This was due to the way redirects work in fetch: the full chain of redirects is followed and you are stuck with the end result. This was resolved using node-fetch and its support for manual redirects.

[linear]

PRO-5666 Logging into Apostrophe back office and then trying to visit the `/login` route results in a 500 error (template not found).