x/pkgsite: Adding function to setup server

[laches1sm]

What changes does this PR bring?

• This PR implements a new function, CreateAndInstallServer(), which refactors duplicated parts of code surrounding the creation and installation of routes and places it into this function.

• For now, I've placed this function within the server.go - please tell me if there's a better place for it!

Thank you all very much in advance for your feedback :D

[gopherbot]

This PR (HEAD: 904b871) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/pkgsite/+/241458 to see it.

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Message from Jonathan Amsterdam:

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[Shellterreur]

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