AddressBook: move subscription saving to storage

[coneiric]

Save subscriptions based on type, and move I/O to AddressBookStorage. ~~~Makes the default hosts.txt read-only.~~~

Resolves #1006

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[anonimal]

#1006 (comment)

[coneiric]

From #1006-comment:

As I've said to you several times already, there is no plan to implement any PGP verification.

Removed it.

Note: the fix is much simpler

This also resolves the TODO in the code to move storage to the AddressBookStorage class. If you would like me to split the PR in two, I can.

[anonimal]

NOTICE: THIS PULL REQUEST HAS BEEN MOVED TO GitLab. Please reopen there if you wish to merge. See #1013 for details.