Fix small css issues [SCI-7699][SCI-7694][SCI-7678]

app/assets/stylesheets/experiment/table.scss

@@ -60,7 +60,7 @@
       position: sticky;
       position: -webkit-sticky;
       top: 0;
-      z-index: 10;
+      z-index: 7;
 
       &.select-all-checkboxes {
         justify-content: center;

app/assets/stylesheets/shared/cards.scss

@@ -58,8 +58,8 @@
       align-items: center;
       display: flex;
       height: 2em;
-      justify-content: center;
-      width: 2em;
+      justify-content: left;
+      width: 1.5em;
     }
   }
 
@@ -111,6 +111,7 @@
 
       .checkbox-cell {
         grid-column: 1;
+        justify-content: center;
         position: initial;
       }
     }

app/assets/stylesheets/shared/content_pane.scss

@@ -91,6 +91,9 @@
               background: $color-concrete;
             }
 
+            .btn {
+              height: 36px;
+            }
 
             a {
               display: inline-block;

app/controllers/my_modules_controller.rb

@@ -35,11 +35,14 @@ class MyModulesController < ApplicationController
   end
 
   def create
-    max_xy = @experiment.my_modules.select('MAX("my_modules"."x") AS x, MAX("my_modules"."y") AS y').take
-    x = max_xy.x ? (max_xy.x + 14) : 1
-    y = max_xy.y ? (max_xy.y + 14) : 1
     @my_module = @experiment.my_modules.new(my_module_params)
-    @my_module.assign_attributes(created_by: current_user, last_modified_by: current_user, x: x, y: y)
+    new_pos = @my_module.get_new_position
+    @my_module.assign_attributes(
+      created_by: current_user,
+      last_modified_by: current_user,
+      x: new_pos[:x],
+      y: new_pos[:y]
+    )
     @my_module.transaction do
       if my_module_tags_params[:tag_ids].present?
         @my_module.tags << @experiment.project.tags.where(id: JSON.parse(my_module_tags_params[:tag_ids]))