[META][UX] Layouts, blocks and contextual links improvements.

[klonos]

While testing #1720, I noticed that when clicking the "configure block" contextual link from the front-end, you are brought to the Layout admin UI with the "Configure block" dialog open for that specific block. This is great UX, but there is room for more improvement there...

• [UX] Add # anchors to blocks in the Layout UI ([UX] Add # anchors to blocks in the Layout UI. #1724).
• [UX] Add a "Remove block" link in the block contextual links (with modal confirmation) ([UX] Add a "Remove block" link in the block contextual links (with modal confirmation). #1725).

[klonos]

...closing in favor of #345 and moving the issues there.