fix(notifications): Verifies that a notification method is registered before using it

Instantly sent notifications now respect the registered notification methods instead
of assuming that a method is available only because it can be found from user's
notification settings.

Fixes #7647
Merge pull request #7648 from juho-jaakkola/verify_notification_methods

fix(notifications): Verifies that a notification method is registered before using it
chore(i18n): update translations
chore(release): v1.9.7
fix(travis): Freezes PHP CodeSniffer on version 1.5.6
chore(travis): Moved PHP_CodeSniffer from pear to composer dependencies
Merge pull request #7678 from Srokap/release-1.9.7

Release 1.9.7