Hoist various imports to the entry point SASS files

In preparation for running the sass-migrator tool, hoist several imports
from included SASS files up to the entry points at
src/styles/{annotator/annotator.scss, sidebar/sidebar.scss}.

src/styles/annotator/annotator.scss

+@import '../variables';
 @import '../base';
 @import '../mixins/icons';
 @import '../mixins/focus';

src/styles/annotator/bucket-bar.scss

-@import '../base';
-
 .annotator-bucket-bar {
   background: rgba(0, 0, 0, 0.08);
   pointer-events: none;

src/styles/base.scss

-@import './variables';
-@import './util';
-
 /* Style input placeholders */
 @mixin placeholder {
   &.placeholder {

src/styles/forms.scss

 // Common form styles.
-@import 'mixins/forms';
 
 .form-field {
   @include pie-clearfix;

src/styles/mixins/forms.scss

-@import '../base';
-
 // See http://compass-style.org/reference/compass/utilities/general/clearfix/#mixin-pie-clearfix
 @mixin pie-clearfix {
   &:after {

src/styles/sidebar/common.scss

-@import '../base';
-@import '../forms';
-
-@import './styled-text';
-
 //ELEMENT STYLES////////////////////////////////
 body {
   position: relative;

src/styles/sidebar/components/annotation.scss

-@import '../../mixins/icons';
-
 // Highlight quote of annotation whenever its thread is hovered
 .thread-list__card:hover .annotation-quote {
   border-left: $highlight 3px solid;

src/styles/sidebar/components/tags-input.scss

 // Styles for the ngTagsInput plugin.
 // http://mbenford.github.io/ngTagsInput/gettingstarted
 
-@import '../../mixins/forms';
-@import '../../variables';
-
 tags-input {
   .host {
     outline: none;

src/styles/sidebar/components/top-bar.scss

-@import '../../base';
-@import '../../mixins/responsive';
-@import '../../mixins/buttons';
-
 .top-bar {
   background: $white;
   border-bottom: solid 1px $grey-3;

src/styles/sidebar/sidebar.scss

@@ -4,7 +4,10 @@ $base-line-height: 20px;
 // Variables and mixins
 // --------------------
 @import '../variables';
+@import '../mixins/buttons';
 @import '../mixins/focus';
+@import '../mixins/forms';
+@import '../mixins/icons';
 @import '../mixins/responsive';
 
 // Base styles
@@ -14,6 +17,9 @@ $base-line-height: 20px;
 
 // Misc. legacy :(
 // ---------------
+@import '../base';
+@import '../forms';
+@import './styled-text';
 @import './common';
 
 // Components