Release plan for Context 7.x-3.8

Focus: PHP 7.2 compatibility. Also address other issues as feasible if RTBC.

Notes

• Drupal 7.6.1, slated for release November 7, 2018, "will feature full compatibility for PHP 7.2". This is a good time for contrib modules to also implement PHP 7.2 compatibility.
• Roughly parallel issue on Features: #3010606: Release plan for 7.x-2.11.

Highest priority items
#2946595: Address PHP 7.2+ compatibility issues

Nice to have
#2196567: Cache query in context_reaction_block::get_blocks() & remove unused blocks before the alters get called.
#1271324: Taxonomy term pages managed by Panels don't trigger Context's "Taxonomy term" condition.