Add login fallback (#3302)

Part of https://github.com/matrix-org/dendrite/issues/3216

The files are basically copied from Synapse, with minor changes to the
called endpoints. We never seem to have had the
`/_matrix/static/client/login/` endpoint, this adds it.

setup/base/static/client/login/style.css

@@ -0,0 +1,79 @@
+html {
+    height: 100%;
+}
+
+body {
+    height: 100%;
+    font-family: "Myriad Pro", "Myriad", Helvetica, Arial, sans-serif;
+    font-size: 12pt;
+    margin: 0px;
+}
+
+h1 {
+    font-size: 20pt;
+}
+
+a:link    { color: #666; }
+a:visited { color: #666; }
+a:hover   { color: #000; }
+a:active  { color: #000; }
+
+input {
+   margin: 5px;
+}
+
+textbox, input[type="text"], input[type="password"] {
+   width: 90%;
+}
+
+form {
+    text-align: center;
+    margin: 10px 0 0 0;
+}
+
+ul.radiobuttons {
+    text-align: left;
+    list-style: none;
+}
+
+/*
+ * Add some padding to the viewport.
+ */
+#container {
+    padding: 10px;
+}
+/*
+ * Center all direct children of the main form.
+ */
+#container > * {
+    display: block;
+    margin-left: auto;
+    margin-right: auto;
+    text-align: center;
+}
+
+/*
+ * A wrapper around each login flow.
+ */
+.login_flow {
+    width: 300px;
+    text-align: left;
+    padding: 10px;
+    margin-bottom: 40px;
+
+    border-radius: 10px;
+    box-shadow: 0px 0px 20px 0px rgba(0,0,0,0.15);
+
+    background-color: #f8f8f8;
+    border: 1px #ccc solid;
+}
+
+/*
+ * Used to show error content.
+ */
+#feedback {
+    /* Red text. */
+    color: #ff0000;
+    /* A little space to not overlap the box-shadow. */
+    margin-bottom: 20px;
+}