Replace hard-coded IDs with configurable ones

The fetched post information is now stored in team name prefixed JSON file,
e.g. tottenhamPosts.json.
Support retrieving posts from Arsenal forum

The Arsenal forum has some differences to the Tottenham one:
- get_config doesn't indicate that the get_forum method is available but it
  still works
- posts don't have a post_count counter, so the logic for determining when
  all posts have been fetched had to be changed
Include post icon URL in stored information
Update stored post information

Include both Tottenham and Arsenal forum posts.