Fix up Store and upgrade Shopify theme

[cjyabraham]

• Currently the red hello bar and red buttons are off-brand
• There are two competing size guide apps
• Make the sold out tag more prominent
• Upgrade the Dawn theme. We're at v. 6.0.2 and the Dawn theme is now up to v 12
• Check out for other things that need improving

[cjyabraham]

I turned the hello bar and red buttons to blue and pink by changing the "accent colors" in the admin.

I removed one of the two size guide apps which caused two competing size guides to show up on products.

I think the sold out tags are ok since when a product is sold out, the regular "Add to Cart" button changes to "Sold Out" and can't be missed.

I'll file the rest of this to Later as upgrading the theme is a significant task.

[cjyabraham]

My sense is that it's not going to be really possible to just upgrade the Dawn theme since we've edited a lot of the theme files. We'd need to re-apply all our edits over again to the updated theme. Is that correct @thetwopct ?

[thetwopct]

We're currently on version 7.0.1 of the Dawn theme. This will continue to work for the time being.

Upgrading the theme to the latest v15 is a significant task due to introduced structural changes, changes to customisations, and changes to variable naming schemas. The biggest difficulties are dealing with the changes made by the locksmith app to various files (locksmith deals with custom page access control). Our theme customisations are mostly confined to the Theme and Header liquid files, and all our CSS is in custom.css (AFAIK), so upgrading is definitely possible alongside extensive testing. But as the theme will continue to work, the upgrade is only needed if recently introduced features or fixes are required.

In this PR, I have made some changes to comply with the shopify theme check, mostly around how assets are loaded/optimisations. This should ensure the theme continues to work until more work is needed.