Update to Blacklight 8

[corylown]

Blocked by #2528

May also be blocked until we address whatever is causing the page editing widget icons not to appear in DLME: sul-dlss/dlme#1880 (more likely related to JS asset handling in DLME)

[corylown]

WIP: main...blacklight8

Known issues:

• Blacklight::SearchBarPresenter is deprecated: Blacklight::SearchBarPresenter is removed in Blacklight 8 #2576
• Skip links: Upgrade classes on skip-links for Blacklight 8 #2575
• Blacklight heatmaps compatibility: Assess compatibility with Blacklight 8 blacklight_heatmaps#63
• Blacklight Advanced Search compatibility. The 8.0.0.alpha2 version of the gem seems to work. The javascript is removed in this version so the reference to //= require 'blacklight_advanced_search' in application.js must be removed.

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