5.0.0 Alpha 13

#2052: Restore (and improve) Pixabay Search
#2055: Fix issue where custom header/footer HTML was not being inserted
#2057: Improved the UX of the "Choose Tags" field
#2058: fix broken installations because of a missing db column
#2004: Adjust sanity checks for last post data
#2049: Remove call to non-existent CSS file
#2051: Skip the call to the parser if the content is empty
#2045: Use Lang::defaultLanguage() when language ID is null
#2046: Rename Warning::content() to avoid signature conflict
#2047: Assassinated the DEBUG_HOOKS constant
#2003: Autogenerate Table Of Contents and Allow Authors to Customize
 - Issue #1765: Table Of Contents widget needs UX TLC
 - Issue #2028: [Alpha12] Widget Area Layout Menu broken in Alpha12
#2025: Recursively clean up quotes
#2034: Use native offsetPosition property
#2035: Added data-ips-member-secondary-groups
#2037: "Custom" formatted page fields are no longer hidden on record feeds
#2039: Fixed alignment of Who's Viewing badge
#2040: Floated images no longer break the alignment of the Edited message
#2041: Improved the UI of the "Course members" table.
#2042: Improved print styles for invoices