Allow subscribe_text to contain block-level elements, fixes #1298

This commit addresses #1298.

The modification will allow the `subscribe_text` field to contain block-level elements, as having it wrapped with a `p` tag was allowing only inline-level elements. In addition to that, use `wpautop()` on the `$subscribe_text` variable for consistency - `wpautop()` is already being used on `$subscribe_text` when displayed on success.