Use mDNS always unless a server has been configured

[mairas]

The WSClient persistence logic now follows the rest of the codebase (I think...):

• If nothing is configured, mDNS is always used.
• If a server is hardcoded, it is used by default.
• If the server address is set through the config page, that address is used.
• If the server address is cleared through the config page, mDNS is used.

Fixes #634.

[mairas]

Seems like I forgot to merge this... Oops!