Password change page improvements

[ArendJan]

I think this is good for now, as it solves our problem when we run the workshops. Two things that we might need to change for future implementations:

1. Maybe change implementation language?

But more imoirtantly:

2. In the current setup we give the ability to change the default password when it is connected to a shared network. So basically anyone can also change the password (since they know the default one). I think the actual check should not be the ability to change the password. I think we should not allow connecting to other networks as long as the password was not changed. This of course is easy for the web interface, but probably harder when connecting to a network through the commandline.

Originally posted by @mklomp in #91 (review)