Center align equations after saving changes on RCE

fixes MAT-459
flag=none

Test Plan:

 - Navigate to an instance of RCE
 - Click on 'Insert'>'Equation' option
 - Insert any short equation you like and then
click on 'Insert Equation'
 - Click on 'Format'>'Align'>'Center' in order to
preview the centered equation image
 - Save changes in the module you're located (i.e quizzes>update question)
 - Now, the equation image should show centered on the preview

NOTES (11-09-2021)

- To apply centering on formulae, it can be done clicking on
formulae image when editing in the RCE, as well as by
selecting the whole p element that contains it
- The recent fix is meant to apply on the preview of the
formulae on Assignments

Change-Id: If7c91458340b60e04bad187a1dd9680be4f91fb1
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/277150
Tested-by: Gonzalo Penaranda <gonzalo.penaranda@instructure.com>
Reviewed-by: Juan Chavez <juan.chavez@instructure.com>
QA-Review: Juan Chavez <juan.chavez@instructure.com>
Product-Review: David Lyons <lyons@instructure.com>

app/stylesheets/base/_ic_utilities.scss

@@ -83,6 +83,8 @@
 .math_equation_latex {
   /*visibility: hidden;*/
   display: inline-block;
+  // adding new style for centering preview of formulae
+  text-align: center;
 }
 .fade-in-equation {
   visibility: visible;

app/stylesheets/components/_g_instructure.scss

@@ -1743,6 +1743,7 @@ form.user_content_post_form {
 }
 
 .equation_image {
+  // Center equation image after saving changes on RCE
   vertical-align: middle;
 }
 

app/stylesheets/our_custom_tiny_mce_stuff/_tinymce.editor_box.scss

@@ -43,6 +43,7 @@ a.instructure_inline_media_comment.mce-item-anchor {
 }
 
 .equation_image {
+  // Center equation image after saving changes on RCE 
   vertical-align: middle;
 }