gitignore: install/logs/*

(Currently they only appear if you've created that directory yourself
for development purposes.  But I have, and it was cluttering my IDE).
migrations controller migrate_to: fix one error message

Don't overwrite the error message if migrations fail!

The error case is probably still broken generally, see #757, but this part
is an obvious coding error.
migrations controller: fix CSRF vulnerability

I must have forgotten about migrate_to() at some point during the audit.

migrate_module() wasn't vulnerable.  But we should make that _obvious_ by
making it look the same.  (It may have been why I forgot about migrate_to).
database controller: CSRF cleanup

These might have been exploitable to DoS the database.

Or in the case of get_backup(), cause strange things to happen by
reading from an arbitrary device file.