Container Registry multi-level image names support

What does this MR do?

Adds support for multiple container images under the same project/group namespace.

Ex: registry.example.com/group/project/any-image-name:optional-tag

Why was this MR needed?

Currently a project's registry can only host a single container image with multiple tags.

While this is appropriate for most use cases and is in line with the 12-factor principles, some valid approaches depend on multiple images. Also, the expected behavior of a Registry is to enable multi-level image names.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#17801

app/assets/stylesheets/pages/container_registry.scss

@@ -3,14 +3,14 @@
  */
 
 .container-image {
-  border-bottom: 1px solid #f0f0f0;
+  border-bottom: 1px solid $white-normal;
 }
 
 .container-image-head {
-  padding: 0px 16px;
+  padding: 0 16px;
   line-height: 4;
 }
 
 .table.tags {
-  margin-bottom: 0px;
+  margin-bottom: 0;
 }