Validation for access point settings

[FernandoGarcia]

User should not be allowed to select the option "never" to disable the access point when there's no valid WiFi credentials saved.
Otherwise the server will stay inaccessible forever and a flash erase will be required.

[thisiscam]

I've made a minimal attempt at this: #279. Feel free to let me know if anything more checks should be added!

[FernandoGarcia]

Hi!

This type of validation is present here.

Best regards.