Bump version to 15.0.0

[tombye]

Includes

• new notification banner format
  #211
  (breaking change)
• changes to temporary messages example page
  #212
• separation of temporary message style into
  placeholder [in
• ability to add pattern-level grids in
  #212

Notes

The new notification banner commit introduced a breaking change.

The base temporary message styles were split out into a placeholder block to allow use across multiple patterns (by extending it):

https://github.com/alphagov/digitalmarketplace-frontend-toolkit/blob/master/toolkit/scss/shared_placeholders/_temporary-messages.scss

This means the existing temporary message pattern will break unless the new placeholder block is included in the main stylesheet of apps that use it.

[TheDoubleK]

Do we want to add a changelog to the toolkit, as we have been introducing elsewhere?
That notwithstanding 👍 for the version bump.

[tombye]

@TheDoubleK Yep, that's a good idea. Now added.