enable current WP site to display read more link to full post on manual excerpts

[xolotl]

WP docs behind this change here: https://developer.wordpress.org/reference/functions/the_excerpt/ http://wordpress.stackexchange.com/questions/141125/allow-html-in-excerpt/141136#141136

[dhwthompson]

Since this code no longer powers the Hypothesis site, I’m closing this issue.