Adding changes for PostGuard encryption

l10n/en_GB.js

l10n/en_GB.json

l10n/nl.js

l10n/nl.json

lib/IMAP/ImapClientRateLimitingDecorator.php

package-lock.json

src/ReplyBuilder.js

src/components/Composer.vue

src/components/ComposerAttachments.vue

src/components/MailvelopeEditor.vue

src/components/Message.vue

src/components/PostGuardEditor.vue

src/components/PostGuardEncryptedBody.vue

src/components/ThreadEnvelope.vue

src/crypto/postguard.js

src/store/actions.js