add several missing alt tags (#8708)

* add several missing alt tags

* remove alt text from mascot images

app/views/about/more.html.haml

@@ -25,7 +25,7 @@
             %span= t 'about.status_count_after', count: @instance_presenter.status_count
         .row__mascot
           .landing-page__mascot
-            = image_tag asset_pack_path('elephant_ui_plane.svg')
+            = image_tag asset_pack_path('elephant_ui_plane.svg'), alt: ''
 
   .column-2
     .landing-page__information.contact-widget

app/views/about/show.html.haml

@@ -63,7 +63,7 @@
                   %span= t 'about.status_count_after', count: @instance_presenter.status_count
               .row__mascot
                 .landing-page__mascot
-                  = image_tag asset_pack_path('elephant_ui_plane.svg')
+                  = image_tag asset_pack_path('elephant_ui_plane.svg'), alt: ''
 
       - else
         .column-2.non-preview
@@ -95,7 +95,7 @@
                   %span= t 'about.status_count_after', count: @instance_presenter.status_count
               .row__mascot
                 .landing-page__mascot
-                  = image_tag asset_pack_path('elephant_ui_plane.svg')
+                  = image_tag asset_pack_path('elephant_ui_plane.svg'), alt: ''
 
       - if Setting.timeline_preview
         .column-3