Remove unused/deprecated code and css

Much of the UI-specific code is no longer part of content-kit-editor, it
being the responsibility instead of downstream consumers such as
ember-content-kit. This commit removes a lot of code that was no longer
being used.

  * Remove unused views, some unused commands
  * remove LinkCommand, Base command, TextFormatCommand
  * Remove unused methods from utils
  * Remove http-utils, only provide 'display' mode for image card
  * Remove unused test helpers
  * remove triggering selection* events
  * Remove 'escapeKey' trigger, favor `didUpdate` instead of trigger('update')
  * move placeholder image src to utils/
  * remove unused css

src/css/application.less

@@ -1,7 +1,2 @@
-@import 'animations';
 @import 'editor';
-@import 'embeds';
-@import 'icons';
-@import 'message';
-@import 'toolbar';
 @import 'tooltip';

src/css/editor.less

@@ -2,7 +2,9 @@
  * Editor
  */
 
- @import 'variables.less';
+// Colors
+@themeColor     : rgb(62, 163, 255);
+@themeColorText : darken(@themeColor, 10%);
 
 .ck-editor {
   font-family: 'Lora', Georgia, serif;