This is an attempt to solve the issue where the publicly-hosted webrepl on https://micropython.org/webrepl is currently non-functional due to the page being served via https, but attempting to connect to a device hosting a non-secure websocket. See #5266

The main details of this change are in the corresponding webrepl client PR -- https://github.com/micropython/webrepl/pull/70

This PR contains:

• Removes unused client_handshake from websocket_helper, and then merges the remainder of this file (server_handshake) into webrepl.py (to reduce firmware size).
• Adds the respond-as-HTTP handling to server_handshake to serve up the <base><script> page.
• Allow the static host to be configured in webrepl_cfg.py (Consider making this part of webrepl_setup flow?)

For testing I've set the default host to https://gitcdn.link/cdn/jimmo/webrepl/webrepl-on-device/ to point to the files from the PR but this should be https://micropython.org/webrepl/ once deployed.