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/lib/FormControl.less

@@ -0,0 +1,42 @@
+.FormControl {
+  display: block;
+  width: 100%;
+  height: 36px;
+  padding: 8px 13px;
+  font-size: 13px;
+  line-height: 1.5;
+  color: @control-color;
+  background-color: @control-bg;
+  border: 2px solid transparent;
+  border-radius: @border-radius;
+  .transition(~"border-color .15s, background .15s");
+  -webkit-appearance: none;
+
+  &:focus,
+  &.focus {
+    background-color: @body-bg;
+    color: @text-color;
+    border-color: @primary-color;
+    outline: none;
+  }
+
+  // Placeholder
+  .placeholder(@control-color);
+
+  // Disabled and read-only inputs
+  &[disabled],
+  &[readonly],
+  fieldset[disabled] & {
+    // background-color: @input-bg-disabled;
+    opacity: 1; // iOS fix for unreadable disabled content; see https://github.com/twbs/bootstrap/issues/11655
+  }
+
+  &[disabled],
+  fieldset[disabled] & {
+    cursor: disallowed;
+  }
+
+  textarea& {
+    height: auto;
+  }
+}