Add possibility to change password for WebDAV mounts

[j-lakeman]

Would be nice to be able to change the passwords for the WebDAV mounts, e.g. if using tokens being revoked after some time.

[ArnyminerZ]

I agree that this can be really useful, also for systems that require a password change once a year, for example.

However, right now we are trying to stabilize DAVx5, so we don't want to add a lot of new features, if they are not imprescindible. We will consider it, though

[j-lakeman]

#539

[devvv4ever]

Yes, we have considered it and made a basic proof, but after evaluating it we pushed it back for several reasons. It will come and it is on our to-do list for the "not-too-distant" future.