Allow author to edit post regardless of access to existing dest. feeds

[davidmz]

When editing a post, access rights to existing destination feeds of the post are no longer checked. This change allows the author of the post to always be able to edit it if he has access. Here are some examples of situations affected by this change:

• The author wrote a post in a public group and his own feed. The group became private, and the author lost access to it, but he still has access to his post. He should be able to edit his post.
• The author sent a direct message, but the recipient deleted his account. The author can no longer send direct messages to him, but he should still be able to edit the existing direct message. This case is particularly important because the author of the direct message cannot delete recipients himself.