Inserter UI

[koke]

A continuation of #58, the inserter works, but we still need it to match the designs.

It was originally estimated as 1 week of effort

[koke]

As reported by @rachelmcr, when we're replacing an existing empty block, the inserter placeholder is still visible saying "Add block here", when it shouldn't.

[koke]

Design update from @iamthomasbishop:

• Block items need to be styled to match designs
• Let’s remove the sheet header to simplify the design for the alpha and beta, then gauge whether it’s a usability concern via feedback. This essentially makes iOS and Android almost entirely aligned, primary difference is iOS uses SF Pro and Android uses Roboto for labels
• Items should be aligned to the grid (see Zeplin for example), 16pt sheet margins left/right, 16pt left/right between items, 12pt bottom margin on each. iOS is a little tricky bc screen widths are odd numbers

[iamthomasbishop]

A note: @etoledom is in the process of implementing RN bottom sheets, so hopefully this can utilize some of that progress: #535