refactor setting a sentence as imported

Instead of calling save again(which triggers an sql-query) we introduce
the `imported` attribute into the hash and set it in the
update_or_create_from_hash method accordingly. Also we refactor the
method to call sentences by their name (sentence), instead of `e`.

Also remove obsolete unused translate_for method.
use short syntax for block-method call.
safeguard against link being nil for translations.
remove unused target-variable.
change imported flag to a two-value field

Instead of the three values, we only want to
allow true and false.
create readable indentation for vanilla sql

squish will remove it on the object level.
refactor sql-reduce for readiblity.