Implement search suggestions so the OpenSearch XML we ship works

[lf-]

Fixes #389.

That wasn't that bad.

This now works in Firefox and Chrome, which is pretty cool.

I tested this locally by hacking up the XML files Hoogle ships:

html/search.xml:

<?xml version="1.0" encoding="UTF-8"?>
<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
    <ShortName>Hoogle - Local</ShortName>
    <LongName>Hoogle - Haskell API Search</LongName>
    <Description>
        Hoogle is a Haskell API search engine, which allows you to
        search many standard Haskell libraries by either function name,
        or by approximate type signature.
    </Description>
    <Tags>haskell</Tags>
    <Url type="text/html" template="http://localhost:8080/?hoogle={searchTerms}"/>
    <Url type="application/x-suggestions+json" template="http://localhost:8080/?hoogle={searchTerms}&amp;mode=suggest"/>

    <Image height="16" width="16" type="image/png">https://hoogle.haskell.org/favicon.png</Image>
    <Image height="64" width="64" type="image/png">https://hoogle.haskell.org/favicon64.png</Image>
    <Developer>Neil Mitchell</Developer>
    <AdultContent>false</AdultContent>
    <Language>en-us</Language>
    <OutputEncoding>UTF-8</OutputEncoding>
    <InputEncoding>UTF-8</InputEncoding>
</OpenSearchDescription>

I'm not sure the best way to do this templating to replace the URLs at present, so I'm leaving that to future work to make that work locally.

[Bodigrim]

@lf- could you please rebase?

[Bodigrim]

I'm a bit worried about bot load, could we make it, say, 5 instead of 25?