Bug 38492: Update occurrences of _() to __()

Bug 38492: Update occurrences of _() to __()

Test plan, ktd:
1) Generate translation files
$ gulp po:update --lang de-DE
2) Verify that strings using _() do not exist on the pot file:
$ cat -n misc/translator/Koha-messages-js.pot | grep "Copied!"
$ cat -n misc/translator/Koha-messages-js.pot | grep "Form not submitted because of the following problem"
$ cat -n misc/translator/Koha-messages-js.pot | grep "No matches found"
3) Apply patch. Repeat test plan.

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>