Fixes for couple of missing translatable messages in e-Newsletter

Couple of missing translatable outputs in e-Newsletter plugin.

e-newsletter-fi/email-newsletter-files/page-create-newsletter.php:

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< jQuery( "#send_preview" ).attr( 'value', 'Send Preview' );
---
> jQuery( "#send_preview" ).attr( 'value', '<?php _e('Send Preview', 'email-newsletter'); ?>' );

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< if ( alt == 'Image Description' ) alt = '';
< jQuery( '#image_alt' ).val( 'Image Description' );
---
> if ( alt == '<?php _e('Image Description', 'email-newsletter'); ?>' ) alt = '';
> jQuery( '#image_alt' ).val( '<?php _e('Image Description', 'email-newsletter'); ?>' );

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< <input type="button" name="image_insert" onclick="jQuery(this).insertImage();" value="Insert Image" />
---
> <input type="button" name="image_insert" onclick="jQuery(this).insertImage();" value="<?php _e('Insert Image', 'email-newsletter'); ?>" />

e-newsletter-fi/email-newsletter-files/page-groups.php:

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< echo "Yes";
---
> _e('Yes');
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< echo "No";
---
> _e('No');