Create _posts directory if it doesn't exist

[greenberga]

Will fix the problem where if all the files in the _posts folder are removed from version control, the folder itself disappears and causes the server to error.

[ERROR] (root) [Errno 2] No such file or directory: '/home/user/blog/_posts'
Traceback (most recent call last):
  File "/home/user/uploader/server.py", line 342, in upload
    new_oid = get_new_oid()
              ^^^^^^^^^^^^^
  File "/home/user/uploader/server.py", line 63, in get_new_oid
    posts = listdir(join(blog_path, '_posts'))
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '/home/user/blog/_posts'
127.0.0.1 - - [22/May/2023 23:42:56] "POST /upload HTTP/1.1" 500 740