Encrypt backup archives v7

[patinthehat]

This PR submits a port of #1239 for v7 (the original PR adds support for encrypting backup archives for for v6). All changes from #1239 have been updated to work with v7.

As with the original PR, there are also some minor updates to the installation-and-setup docs.

Resolves #1245.

@freekmurze @Gummibeer

[freekmurze]

Looks good! Have you also tested this manually, just to be sure that you can decrypt the zips with the right password?

[patinthehat]

Looks good! Have you also tested this manually, just to be sure that you can decrypt the zips with the right password?

Yes. I was able to decrypt the zips with the correct password without any issues.

[Gummibeer]

As you've asked: for me as Mac user the only problem was that the default Archive/Zip application on macOS isn't able to extract encrypted archives.
So I had to use another application (Unarchiver) but it was the same for all archives, so seems like a problem with the app and not the packages logic.

[freekmurze]

Thank you very much!

[drbyte]

I tested it on the v6 branch and BetterZip (Mac) can read the archives fine after prompting for the password.
Clarification: zips created and encrypted on ubuntu server via this package; and I unzipped on my Mac using BetterZip

[Gummibeer]

Yeah, it's only the default program used by right-click "compress" on a folder too.
It just fails with an error and says that the ZIP is corrupted - which isn't the case! The program is simply unable to open encrypted archives. 😂

Possibly worth a note on the docs => will send PRs.

[drbyte]

default Archive/Zip application on macOS isn't able to extract encrypted archives

I confirm the same symptom using the default macOS Archive application. (In my case macOS 10.14/Mojave.)

(It can handle unencrypted archives fine. I'm certain this is not an issue with this package's encryption feature.)