Improve I18N issues

[alexclassroom]

• This plugin's slug is wp-cfm, but the current UI translatable strings with the wrong Text Domain is wpcfm. Change the wrong Text Domain so it is equal to this plugin's slug and modify the text domain in all your source files. This change is needed, please refer to this article.
• For security reason, change <?php _e( to <?php esc_html_e(. Please refer to this official article.
• Make the UI strings without I18N code translatable.

[timnolte]

FYI, this was partly taken care of in #145 .