Fix escaping in post summary

[Antolius]

Unescaping post content after sanitizing it. This will prevent double
escaping when summary is rendered by html/template package which does
escaping by default.
Fixes #340

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[Antolius]

The double encoding issue could also be fixed by updating templates to use the SummaryHTML method here and here.

I opted to unescape Content in the Summary method because Title is already treated that way so this seemed more in line with the existing code. If you'd prefer updates to the templates instead, I can switch it up.

[thebaer]

Thanks for addressing this, @Antolius! I think the route you chose for this solution was the right one. Merging now!