[3.6.x] Use username & password from UI to log in to Plex

Currently Plex login ignores the username and password from the UI and instead always looks up the Keychain for credentials. This will fail if no credentials have been stored, or if the stored credentials are incorrect. This will also result in unexpected behavior if the user wishes to log in with different credentials on the same server. This change means the code will attempt to log in with the given username and password before falling back to credentials stored in the Keychain.

Closes #1824 (closed)

(cherry picked from commit 92bd9542, backport of !1468 (merged))