Apps can say they work with Gearslip, and each recommended app gets its own page

An app adds one meta-data line to its manifest and Gearslip marks it Works with Gearslip in the
Recommended apps list. Tapping an app opens its page, with an Open or Get button. A README badge
for app developers is in docs/badge, documented in BUILDING_APPS.md.

docs/badge/works-with-gearslip.svg

@@ -0,0 +1,16 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="147" height="20" role="img" aria-label="Works with Gearslip">
+  <title>Works with Gearslip</title>
+  <clipPath id="r"><rect width="147" height="20" rx="3" fill="#fff"/></clipPath>
+  <g clip-path="url(#r)">
+    <rect width="88" height="20" fill="#555"/>
+    <rect x="88" width="59" height="20" fill="#8C1F1F"/>
+  </g>
+  <g transform="translate(5 3.5) scale(0.228) translate(-29.5 -25.5)" fill="#fff">
+    <path fill-rule="evenodd" d="M54 36.18 58.64 30.7 67.2 34.24 66.6 41.4 71.82 54 77.3 58.64 73.76 67.2 66.6 66.6 67.2 73.76 58.64 77.3 54 71.82 49.36 77.3 40.8 73.76 41.4 66.6 34.24 67.2 30.7 58.64 36.18 54 30.7 49.36 34.24 40.8 41.4 41.4 40.8 34.24 49.36 30.7ZM54 46.08A7.92 7.92 0 1 1 53.99 46.08Z"/>
+    <path d="M78.61 40.7 85.31 43.29 84.74 52.55 77.78 54.31Z"/>
+  </g>
+  <g fill="#fff" font-family="Verdana,Geneva,DejaVu Sans,sans-serif" font-size="11" text-rendering="geometricPrecision">
+    <text x="22" y="14" textLength="60">works with</text>
+    <text x="94" y="14" textLength="47">Gearslip</text>
+  </g>
+</svg>