FIX: Allow for custom admin url

[PortableSteve]

Description

Allow inline editable content blocks to be saved when using a custom admin route.

Manual testing steps

• Set a custom admin route.
• Add a Content Block to an element area.
• Make changes to the content block and try to save it.
• Verify that changes are saved successfully.

Issues

• 5.3 - inline editable blocks broken with custom admin route #1281

Pull request checklist

• The target branch is correct
• All commits are relevant to the purpose of the PR (e.g. no debug statements, unrelated refactoring, or arbitrary linting)
• The commit messages follow our [commit message guidelines]
• The PR follows our contribution guidelines
• Code changes follow our coding conventions
• This change is covered with tests (or tests aren't necessary for this change)
• Any relevant User Help/Developer documentation is updated; for impactful changes, information is added to the changelog for the intended release
• CI is green

[michalkleiner]

Thanks for the PR @PortableSteve, can you please rebase/retarget to 5.3 so it can go out in the next patch release? Thanks!

[PortableSteve]

Thanks for the PR @PortableSteve, can you please rebase/retarget to 5.3 so it can go out in the next patch release? Thanks!

Done, thanks!

[michalkleiner]

The change looks good to me, over to @GuySartorelli to merge on green unless there are any other changes/comments.

[GuySartorelli]

LGTM. Michal you do know you're allowed to merge things right? :p