FIX: encrypt works for a user who cannot edit posts (PR #24)

The bug mentioned here: 'min trust to edit post' setting interfering with Discourse Encrypt - bug - Discourse Meta

Using the guardian was the best and safest approach. However, there is a situation when the user cannot edit posts. For example when min_trust_to_edit_post is set higher than the user trust level. Therefore we can simply check if the user is author of the post.

GitHub

If edit isn’t the appropriate action do we need a guardian.ensure_can_encrypt_post! instead?