Mostly to avoid long restore times thanks to an index not being built
before it's needed by restoring the DB schema first.
https://blog.freespeechextremist.com/blog/activities-visibility-index-slowness.html
Also updates backup command to compress DB backups, removes Pleroma
users's home directory, replaces "role" with "user" in PostgreSQL contexts
since they are the same now.
* Added "Migrate" to the title because these steps can also be used to migrate the instance to another server
* Added an optional step to reinstall pleroma (esp. for migrating servers)
* Currently the steps threw an error 'could not execute query: ERROR: function "activity_visibility already exists with the same argument types'
* I added a new step to drop and recreate an empty pleroma-database
* I played around with the `-c` and `-C` options of pg_restore, but dropping and recreating seemd to be the only way I got it working
* This was tested on Debian Stretch, psql (PostgreSQL) 9.6.15
