External link marker: new tab + icon on links leaving the blog

External links in post bodies now open in a new tab and show a
box-with-arrow marker icon, so readers can tell at a glance which
links leave the site.

- blogex: new Blogex.ExternalLinks.rewrite/1 marks up anchors with
  http(s) hrefs at compile time with target="_blank", rel="noopener"
  and class="blogex-external" (idempotent, merges into existing
  class/rel values, leaves relative, mailto: and #anchor links alone)
- blogex: Post.build/3 applies the rewrite, so every blog gets it
- app: CSS in .blogex-post-body draws the marker via a mask so it
  inherits the link colour; empty ::after content stays invisible
  to screen readers
- make test now also runs the blogex suite; it is currently red
  with 3 pre-existing blog_integration_test failures caused by the
  vacuous link validator (parked yak
  link-validator-never-sees-compiled-html-...)

blogex/Makefile

@@ -0,0 +1,12 @@
+# Makefile for Blogex library
+
+MISE_BIN ?= $(HOME)/.local/bin/mise
+MISE_EXEC = $(MISE_BIN) exec --
+
+.PHONY: test format
+
+test:
+	$(MISE_EXEC) mix test
+
+format:
+	$(MISE_EXEC) mix format