remove hard-coded pushover api token, require one from the user

The hard-coded Pushover API token keeps hitting its monthly limit.  We (Superblock) would prefer to have each user register his or her own application to get a new quota of 7500 notifications per month, rather than having every user on the same token.

This removes the hard-coded token and adds support for getting the token from the user.

This is a mechanical change and has not been tested or even compiled.