Use filename as id/class prefix. (#16997)

Currently the svg minifier (`make svg`) rewrites all `id` and `class` attributes in svg files. Every file gets the ids `a, b, ...`. If multiple svgs with ids are used on a page these ids are conflicting and the results are broken images.

| minified ids | unique ids |
| - | - |
| ![grafik](https://user-images.githubusercontent.com/1666336/132579375-59d3996f-c4e5-43b8-8c8d-82280c90d9e3.png) | ![grafik](https://user-images.githubusercontent.com/1666336/132579413-05bf9285-4e3b-4d0d-8f95-90b212405b05.png) |

This PR adds a prefix (the filename) to every id/class.

Follow up problem: Because we embed svg images there are duplicated ids if one svg image is used multiple times on a page. As those ids refer to the same content it may be no real problem because browser handle that fine.

build/generate-svg.js

@@ -30,6 +30,7 @@ async function processFile(file, {prefix, fullName} = {}) {
       {name: 'preset-default'},
       {name: 'removeXMLNS'},
       {name: 'removeDimensions'},
+      {name: 'prefixIds', params: {prefix: () => name}},
       {name: 'addClassesToSVGElement', params: {classNames: ['svg', name]}},
       {name: 'addAttributesToSVGElement', params: {attributes: [{'width': '16'}, {'height': '16'}, {'aria-hidden': 'true'}]}},
     ],

public/img/svg/gitea-github.svg

@@ -1 +1 @@
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+<svg viewBox="0 0 64 64" class="svg gitea-github" width="16" height="16" aria-hidden="true"><linearGradient id="gitea-github__a" x1="30.999" x2="30.999" y1="16" y2="55.342" gradientUnits="userSpaceOnUse" spreadMethod="reflect"><stop offset="0" stop-color="#6dc7ff"/><stop offset="1" stop-color="#e6abff"/></linearGradient><path fill="url(#gitea-github__a)" d="M25.008 56.007c-.003-.368-.006-1.962-.009-3.454l-.003-1.55c-6.729.915-8.358-3.78-8.376-3.83-.934-2.368-2.211-3.045-2.266-3.073l-.124-.072c-.463-.316-1.691-1.157-1.342-2.263.315-.997 1.536-1.1 2.091-1.082 3.074.215 4.63 2.978 4.694 3.095 1.569 2.689 3.964 2.411 5.509 1.844.144-.688.367-1.32.659-1.878-4.956-.879-10.571-3.515-10.571-13.104 0-2.633.82-4.96 2.441-6.929-.362-1.206-.774-3.666.446-6.765l.174-.442.452-.144c.416-.137 2.688-.624 7.359 2.433a24.959 24.959 0 0 1 6.074-.759c2.115.01 4.158.265 6.09.759 4.667-3.058 6.934-2.565 7.351-2.433l.451.145.174.44c1.225 3.098.813 5.559.451 6.766 1.618 1.963 2.438 4.291 2.438 6.929 0 9.591-5.621 12.219-10.588 13.087.563 1.065.868 2.402.868 3.878 0 1.683-.007 7.204-.015 8.402l-2-.014c.008-1.196.015-6.708.015-8.389 0-2.442-.943-3.522-1.35-3.874l-1.73-1.497 2.274-.253c5.205-.578 10.525-2.379 10.525-11.341 0-2.33-.777-4.361-2.31-6.036l-.43-.469.242-.587c.166-.401.894-2.442-.043-5.291-.758.045-2.568.402-5.584 2.447l-.384.259-.445-.123c-1.863-.518-3.938-.796-6.001-.806-2.052.01-4.124.288-5.984.806l-.445.123-.383-.259c-3.019-2.044-4.833-2.404-5.594-2.449-.935 2.851-.206 4.892-.04 5.293l.242.587-.429.469c-1.536 1.681-2.314 3.712-2.314 6.036 0 8.958 5.31 10.77 10.504 11.361l2.252.256-1.708 1.49c-.372.325-1.03 1.112-1.254 2.727l-.075.549-.506.227c-1.321.592-5.839 2.162-8.548-2.485-.015-.025-.544-.945-1.502-1.557.646.639 1.433 1.673 2.068 3.287.066.19 1.357 3.622 7.28 2.339l1.206-.262.012 3.978c.003 1.487.006 3.076.009 3.444l-1.998.014z"/><linearGradient id="gitea-github__b" x1="32" x2="32" y1="5" y2="59.167" gradientUnits="userSpaceOnUse" spreadMethod="reflect"><stop offset="0" stop-color="#1a6dff"/><stop offset="1" stop-color="#c822ff"/></linearGradient><path fill="url(#gitea-github__b)" d="M32 58C17.663 58 6 46.337 6 32S17.663 6 32 6s26 11.663 26 26-11.663 26-26 26zm0-50C18.767 8 8 18.767 8 32s10.767 24 24 24 24-10.767 24-24S45.233 8 32 8z"/></svg>