Remove 'coreconcepts-' prefix from styles, admonition blocks, etc

[pdaoust]

There are some template elements called coreconcepts-intro, coreconcepts-orientation, etc. The naming is a legacy from when they were only used on the core concepts pages, but now they're getting used in other places too.

There are a couple places where they show up:

• The Markdown parser has been extended to recognise them as special blocks that deserve their own styling
• There are matching CSS styles

This is just a simple search-and-replace. Update: see below. I'm pretty sure the shortened names don't clash with anything.

Update

I found out the styles weren't hooked up to these block containers anyway, so I've done some work to make them work. You can see all three at work in e.g., https://deploy-preview-438--developer-portal-production.netlify.app/concepts/7_validation/

One big change is that the big ::: storysequence block has been changed to more verbose, but much simpler and less magical, ::: storystep blocks around each step in a story sequence. I don't think we'd ever reimplemented the original site's behavour, in which each step was a list item.

[pdaoust]

okay, this is weird... I just discovered that these special Markdown blocks get custom markup and styles, but never the twain do meet -- the styles live in a file that never gets loaded by any page!

[mattyg]

Noted a few potential styling tweaks (I don't have a good design sense so feel free to ignore).

Also I see that the Get Started page and Resources page have "intro" text that isn't tagged for the intro styling.