Ignore keepalive file when restoring backup

[matthew-white]

Right now lib/bin/restore.js quits with the following error message when restoring a backup downloaded from /v1/backup:

Required parameter undefined missing.
'keepalive local key'

This PR modifies the restore process so that it ignores keepalive.

I had trouble writing a test for this change. Ideally, we would test the result of calling decryptFromArchive() on a .zip file that contains both encrypted files and a keepalive file. encryptToArchive() returns a .zip files with encrypted files, but I didn't see an easy way to insert a keepalive file into it. Instead, I verified this change as follows:

• I downloaded a .zip file locally using the "Download backup now" button.
• I modified restore.js so that it expands the zip into a persistent directory and decrypts encrypted files, but doesn't run pgrestore().
• I ran restore.js. It did not show the error message and expanded a number of files.
• I committed those files to a Git repo.
• Next, I ran lib/bin/backup.js, uploading a backup to Google Drive.
• I downloaded that file and ran the modified restore.js on it. I moved the resulting files into the Git repo, comparing to the immediate backup.
• The only file that was different was toc.dat, a binary file.

[lognaturel]

Looks right to me! Have you had a chance to try it as a sanity check?

[matthew-white]

Yes, just added some notes above about how I tried to verify!