Refactor to reduce complexity of Post#save_reply_relationships

Continuing to refactor complex methods of Post.

The following are the results of the changes in this PR:

|         |      Post       | save_reply_relationships  |
| ------------- | -------------:| -----:|
| flog score before      | 560 | 35.7 |
| flog score after | 550 |  13.2 |