Issue #3318549 by smustgrave, mstrelan, larowlan, rkoller, AaronMcHale, acbramley: Rename Custom Block terminology in the admin UI

Issue #2512456 by tim.plunkett, legolasbo, MattA, dawehner, googletorp, darol100, Bojhan, webchick, andypost, ivanstegic, larowlan, lauriii, LewisNyman, tkoleary, rickvug, eliza411, lunk_rat, nickrosencrans, stpaultim, Mark LaCroix: Implement the new block layout design to emphasize the primary interaction of placing a block

Issue #1920862 by martin107, Mixologic, cilefen, larowlan, xan.ps, tim.plunkett | sun: Rename custom_block.module to block_content.module.