Can't add a new server to an existing user in FlowAuth

[jc-harrison]

Describe the bug

If I log into FlowAuth as an admin user, add a new server, and then try to add the new server to an existing user, I get re-directed to the login screen and the changes don't take effect.

To Reproduce

1. Spin up FlowAuth in demo mode, and log in as 'TEST_ADMIN'.
2. In the 'Servers' tab, add a new server 'NEW_SERVER'.
3. Select the 'Users' tab, and edit 'TEST_USER'.
4. In the 'Servers' drop-down, select 'NEW_SERVER'.
5. Select 'Save'.
6. The login screen appears.
7. Log in as 'TEST_USER'
8. 'TEST_SERVER' is still the only available server.

Expected behavior

When a new server is added through the FlowAuth UI, it should then be possible to give an existing user permissions for this server.