First phase of Digital Publication improvements

Added/improved:

• Update Digital Publication Article and Digital Publication Article CMS form to account for expanded layout [PUB-196, PUB-197]
• Rename instances of Digital Publications Sections to Articles [PUB-191]
• Allow editors to select a hero color per publication [PUB-192]
• Create "tombstone" block for digital publications [PUB-187]
• Build a foundation for theming in Digital Publications [PUB-186]
• Allow editors to add hierarchy to articles in digital publications [PUB-193]
• Update the table of contents on frontend to handle the hierarchy of articles [PUB-199]
• Add a "ranged accordion" block to collapse a range of block content into an accordion [PUB-195]
• Create 'Entry' article type for Digital Publication Articles [PUB-190]
• Add 'Contribution' article styles for Digital Publication Articles [PUB-191, PUB-201]
• Populate body of landing page with abridged grouped content in digital publications [PUB-206]
• Create listing page with all grouped content in digital publications [PUB-205]
• Send an email when an exhibition is automatically moved from "upcoming" to "current" [WEB-2884]
• Add teal showcase block variation to landing pages [WEB-2882]
• Update copy for Educator Admission Request [WEB-2894]

Fixed:

• Add graceful fallback for 360º blocks when no images are selected [PUB-212]
• Fix the layout of megamenu when the disruptor bar is displayed [WEB-2893]
• Fix the layout of megamenu when the header isn't sticky in the browser window [WEB-2903]
• Fix bug preventing CMS previews of Interactive Features [WEB-2886]
• Fix sorting by "Is DSC stub?" column [PUB-196]
• Minor visual QA on RLC landing page [WEB-2868]
• Fix bug causing remove item in My Museum Tour sending two actions to GTM [WEB-2861]
• Fix display of medium and large images in digital publications [PUB-217]
• Display article date when there are no authors [WEB-2902]