Change permissions on backup files - #2

Use more restrictive permissions for backup tar files and for the db, uploads, and repositories directories inside the tar files.  See #1894.  Now the backup task recursively `chmod`s the `db/`, `uploads/`, and `repositories/` folders with 0700 permissions, and the tar file is created as 0600.

This is a followup to !1703, which was reverted because it broke Rspec tests.  The test failures were due to the rake task changing directories and not changing back, which I fixed with this commit.

cc @sytse

See merge request !1716