add share link at /m/, add share dialog in view menu, fix preview for documents

static/mobile/js/api.js

@@ -0,0 +1,25 @@
+var pandora = {
+    format: getFormat(),
+    hostname: document.location.hostname || 'pad.ma'
+}
+
+var pandoraURL = document.location.hostname ? "" : `https://${pandora.hostname}`
+var cache = cache || {}
+
+async function pandoraAPI(action, data) {
+    var url = pandoraURL + '/api/'
+    var key = JSON.stringify([action, data])
+    if (!cache[key]) {
+        var response = await fetch(url, {
+            method: 'POST',
+            headers: {'Content-Type': 'application/json'},
+            body: JSON.stringify({
+                action: action,
+                data: data
+            })
+        })
+        cache[key] = await response.json()
+    }
+    return cache[key]
+}
+