Learn English – What does “potted” mean in “Potted Potter”

As stated by yourself, Harry doesn't like anyone to be interested in the Daily Prophet.

When "Harry" said 'you and the rest of the world', he is stating : "Everyone reads the Daily Prophet!"

It's the same when someone say, teases you. You could get upset and say "No one likes me anymore!"

Not really literally no one. It's just an expression.

"He won't know himself" means that he'll have changed so much because he has learned so much skills, that he would practically be unrecognizeable even to himself.

Note the previous sentence, "He's off ter the finest school of witchcraft and wizardry in the world." then, the next statement says that after he's been there seven years, he'll have changed so much, for the better, that he wouldn't be able to tell what he was like before, he wouldn't even recognize himself.

Other examples of usage of "won't know himself" could be:

It's the best beauty parlour in the world. After she's been there, she wouldn't know herself.

The seminar's great. After you attend, you won't know yourself.