Remove StackImpact showcase

Closes #1719

content/en/showcase/stackimpact/bio.md

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-**StackImpact** is a production-grade performance profiler for production and development environments.
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-The [stackimpact.com](https://stackimpact.com/) website is built with awesome Hugo. Grunt is used for CSS minification and bundling.

content/en/showcase/stackimpact/index.md

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----
-title: StackImpact
-date: 2018-02-20
-description: "\"Hugo is a perfect choice for a product website.\""
-siteURL: https://stackimpact.com/
----
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-After gradually handing over the control of our website to WordPress plugins, we realized that we needed to act.
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-Static web sites have natural advantages such as security, performance and content versioning (e.g. with Git). Static site generators such as Hugo made static websites cool again. Importantly, the best practices of software development (e.g. peer reviews, multi-stage deployments, rollbacks) can now be easily applied to websites.
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-Besides the blog and documentation sections, our website needed custom sections with own templates. Hugo supported it beautifully.
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-Hugo is written in Go language and uses Go templates. StackImpact is a performance profiler that has an advanced support for Go applications. Being aware of the advantages of Go in terms of speed and productivity, this was another strong reason for choosing Hugo.
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-Thanks to all Hugo contributors for making such a beautiful and fast site generator!