Update Inserter toggle icon style

[iamthomasbishop]

The Inserter toggle icon ( (+) ) is using the outlined icon in gray. We need to update the following:

• Use filled icon variation: The original design proposed using Gridicons (which we're using elsewhere in the apps), but Material icons also offer a pretty much exact alternative ("add_circle"), but I'm not sure if we have easy access to that via Core. If not, we should either propose adding that to the Core set, or add it manually for now.

• Change the icon color to the Primary WordPress blue that's in use: $blue-wordpress aka #0087be.

It looks like this:

Related to: #55