Make all hotlinked images local

[Kestrer]

This closes #578 and half of #579.

The only remaining image hotlink is 2016-04-19-MIR, which has two references to hotlinked images in its Markdown that aren't used anywhere in the article.

[rust-highfive]

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[XAMPPRocky]

@Koxiaet Thank you for your PR! Is there a reason this PR doesn't also remove the hotlinks in the MIR post?

[Kestrer]

@XAMPPRocky As it wasn't used I felt that it might be the job of another PR to remove all unused links in blog posts.

[XAMPPRocky]

@Koxiaet Oh I didn't see the "aren't used anywhere in the article." part. It seems fine to leave then as it won't affect the readers.

Thank you again for your PR, and congrats on your first contribution!