Custom markup container default to space between

client/src/app/shared/shared-custom-markup/custom-markup.service.ts

@@ -217,6 +217,8 @@ export class CustomMarkupService {
 
     root.classList.add('peertube-container', layoutClass)
 
+    root.style.justifyContent = data.justifyContent || 'space-between'
+
     if (data.width) {
       root.setAttribute('width', data.width)
     }

shared/models/custom-markup/custom-markup-data.model.ts

@@ -53,4 +53,6 @@ export type ContainerMarkupData = {
   title?: string
   description?: string
   layout?: 'row' | 'column'
+
+  justifyContent?: 'space-between' | 'normal' // default to 'space-between'
 }