[Gutenberg] Make block icons more specific to the blocks

[postphotos]

[Gutenberg] As an author, I need block icons to better contextualize what the custom blocks mean AC1. The custom blocks now all have ‘WP’ icons, so change them to something more specific. For example, the ‘Search’ block’s registerBlockType() could have an icon of ‘search.’ https://developer.wordpress.org/resource/dashicons/

@postphotos - Proposed nice to have: AC2. Identify all of the blocks that we've created AC3. Design or select icons to create visuals that better match those not served by dashicons images.

As requested by @kienstra

(p.s. @kienstra's PR #47 already addresses this)