Major CSS revamp

- Get rid of Bootstrap (except we still rely on some JS)
- Use BEM class names
- Rework variables/theme config
- Fix various bugs, including some on mobile

The CSS is still not ideal – it needs to be cleaned up some more. But
that can be a focus for after beta.

less/forum/SignUpModal.less

@@ -0,0 +1,44 @@
+.signup-welcome {
+  position: absolute;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  border-radius: @border-radius;
+  padding: 50px 30px;
+  text-align: center;
+  color: #fff;
+  font-size: 14px;
+
+  .light-contents();
+
+  & .avatar {
+    .Avatar--size(96px);
+    border: 4px solid @body-bg;
+    .box-shadow(0 2px 6px @shadow-color);
+  }
+  & h3, & p {
+    margin-bottom: 25px;
+  }
+  & .btn-default {
+    font-size: 15px;
+    height: 50px;
+    padding: 15px 20px;
+  }
+  & .container {
+    width: auto !important;
+    padding: 0 !important;
+    position: relative;
+  }
+}
+.signup-modal-success {
+  & .close {
+    color: #fff;
+  }
+}
+
+@media @phone {
+  .signup-welcome {
+    padding-top: 56px + 60px;
+  }
+}