fixed broken oEmbed for laterpay_teaser_content #403

* fixed not working oEmbed on laterpay_teaser_content
* removed wpautop() from save laterpay_teaser_content
* added new priority for the `the_content`-Filter to modifiy as first the `post_content`
* fixed broken purchase-link and purchase-button on wpautop() (no new lines in attributes!)

References to: #403