Adds migration for invalid attachable journals

Invalid attachable journals are those without an initial attachable
entry.
Refactors attachable repair migration
Adds message attachment spec
Fixes inital message attachment journal issue
Repairs messages' initial attachment journal
Removes superfluous let statement
Removes superfluous and wrong comment
Adds message add/remove attachment specs
Fixes message attachment add/remove
Adds migration for message attachment journals
Adds changelog entry