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');