Issue#162 create endpoint to get random posts

[oguzhekim]

Created endpoint: GET /api/posts/random?count=x to get random posts. This endpoint doesn't require authentication. Refactored post endpoints. You should not send user id when creating a post from now on, the user is decided from authentication header. Delete post endpoint checks if the authenticated user is the owner of the post.

[oguzhekim]

Two more changes:

• PostResponse returns username rather than userId.
• Refactored getAllPosts function in PostService.