Design tweaks to dialogs

Little bit of a mix of things in this one:
 * Support variable-width dialogs. Default is fixed-width as before,
   only UploadConformDialog is variable-width. Controlled by a prop
   to BaseDialog.
 * Fixes to the cancel 'x' - scale the mask image, tweak size & colour
 * Colour & boldness of dialog titles
 * Align the dialog title & cancel 'x'
 * Remove gap between dialog buttons & right hand side of dialog(!)
 * Round corners on dialogs
 * Add grey border on image preview in upload confirm dialog
 * and, squeezing in slightly randomly, finish the partially renamed
   ChatInviteDialog to AddressPickerDialog.

res/css/views/dialogs/_SettingsDialog.scss

@@ -21,7 +21,7 @@ limitations under the License.
         height: 80%;
         border-radius: 4px;
         padding-top: 0;
-        padding-right: 0;
+        padding-right: 30px;
         padding-left: 0;
 
         .mx_TabbedView {
@@ -31,7 +31,7 @@ limitations under the License.
         .mx_TabbedView .mx_SettingsTab {
             box-sizing: border-box;
             min-width: 580px;
-            padding-right: 130px;
+            padding-right: 100px;
 
             // Put some padding on the bottom to avoid the settings tab from
             // colliding harshly with the dialog when scrolled down.