[Feature/only op can view] Add a permission to phpBB that allows viewing other user's posts

This adds a new permission to phpBB I called f_read_other. What this does is to allow the user to read threads that were started by another member of the forum which is not himself. Of course, there are exceptions.
This does not effect Global topics and Announcements topics (but does affect stickies).

But... why making it like this, as a permission?
To sum it, it's more flexible to have it this way
E.g: There's not need to be a moderator in order to have that user able to view all posts.

With the roles that come with this addition it should not be harder to use this than it was before.

PHPBB3-10623