Implement 360Giving Design System

[robredpath]

• It's clear how the world will be different when the work is completed

All existing pages in GrantNav will use the new theme except those which are being removed.

• It's clear what will and won't be done

Apply the 360Giving Design System to all pages on GrantNav, once this PR is merged

@robredpath will provide a list of pages which are being removed.

• Any uncertainty, assumptions, dependencies or constraints are recorded

• There's an order-of-magnitude estimate of how long it will take

Days

• The work's been approved by someone at ThreeSixtyGiving

• There are links to everything that's written down about the work - such as plan.io tickets, GitHub issues

[robredpath]

@Lathrisk I've realised while prepping for #601 that we don't have anything already designed for the "tip" that appears in certain circumstances - such as if you carry out a multi-word search

Do you feel like you've got enough from the rest of the design system to incorporate that, or do we need to go back to agile.coop on that? (cc @drkane )

[drkane]

That's a good point - there was definitely a version of this in the mockups at one point.