Admin pages that are based off the customize or plugin templates were wrapped in the `admin-container` class, which has slightly different margins to the `admin-config-page` class, which is used on most admin pages.
This change ensures that all the admin pages have the same alignment.
There are a few changes here to make the Emails admin page more consistent with the rest of the admin UI.
- The header and navigation menu have been updated.
- The sidebar now stays highlighted when visiting the email admin sub-pages.
- Moved the Template editor from /admin/customize/email_templates to /admin/email/templates, so it fit as a sub-page.
- Removed the link to the Template editor from the Customize section of the old top menu, since it's accessible from the Emails section, instead.
To add a components link to the sidebar refactoring was required to create unique URLs for themes and components. Before the query param was used. After changes, we have two URLs `/admin/customize/themes` and `/admin/customize/components`.
