Display reusable blocks in the inserter's 'Recent' tab

[noisysocks]

🐶 What this is

Closes #4653.

Shows reusable blocks in the 'Recent' tab of the inserter.

🤔 How this works

Before, our reducer would build out a state.preferences.recentlyUsedBlocks that looks like this:

recentlyUsedBlocks: [ 'core/paragraph', 'core/image' ],

This PR changes the shape of this data to look like this:

recentInserts: [
    { name: 'core/paragraph' },
    { name: 'core/image' },
    { name: 'core/block', ref: 123 }
],

It's then just a simple matter of patching selectors.getRecentInserterItems to look up reusable blocks by the ref that is stored.

I've renamed recentlyUsedBlocks to recentInserts so that we don't need to worry about parsing old data which is in a different shape. This means that, when a user upgrades to the next version of Gutenberg, the blocks that appear in their 'Recent' tab will be reset. I think this is an acceptable trade-off since this data isn't all that important and will be quickly re-populated.

📋 How to test

1. Insert a regular block
2. Insert a reusable block
3. Open the inserter and see that both blocks appear in the 'Recent' tab
4. Check that clicking on the block in the inserter inserts that block

[youknowriad]

minor: maybe just my obsession for shorter code. This could be:

const insert = { name : block.name };
if ( isReusableBlock( block ) ) {
  insert.ref = block.attributes.ref;
}

[noisysocks]

Changed in 6d77903. All this newfangled syntax has made me forget that mutation is still a thing 😆

[youknowriad]

It works great in my testing, nice work 👍