Tick a guide's sections off as you work through it

Only checklist items could be ticked, which answers "what have I done" but not
the question a long procedure actually raises: where was I. Somebody looks away
to do the thing the guide asked for and looks back at a wall of text.

Every step and every heading now carries a checkbox while reading. Ticking one
strikes it through and fades it rather than hiding it, because somebody
scanning back up the page has to still be able to read what they did. The
number is struck through with it, but the step's own "Step 3" label is not -
that is the address of the thing, not part of what was done.

Headings count, not just steps. A guide converted from an HTML article often
has fifty headings and no steps at all, and those are exactly the guides long
enough to lose your place in. That also means the card's "N steps to tick"
stops reading as a broken feature on a guide that is entirely tickable.

Reading only, as the checklists already were: in the editor a tick would be an
edit, and an author clicking around their own draft would quietly change what
the next reader sees as done. Progress stays per browser and per version -
shopfloor PCs share a login, so one technician's ticks must not greet the next
person on that bay, and step 4 of version 3 is not step 4 of version 2.

CHANGELOG.md

@@ -12,6 +12,18 @@ ADR-007 and ADR-002.
 
 ### Added
 
+- **A guide's sections can be ticked off while you read it.** Only checklist
+  items were tickable, which answered "what have I done" but not the question a
+  long procedure actually raises - where was I. Somebody looks away to do the
+  thing the guide asked for and looks back at a wall of text. Every step and
+  every heading now carries a checkbox in the reader; ticking one strikes it
+  through and fades it rather than hiding it, because scanning back up the page
+  has to still show what was done. Headings count as well as steps: a guide
+  converted from an HTML article often has fifty headings and no steps at all.
+  Progress is per browser and per version, as it already was for checklists -
+  shopfloor PCs share a login, so one technician's ticks must not greet the
+  next person on that bay.
+
 - **A guest is a role now, not a switch per feature.** Whether somebody with no
   account may see something was answered one feature at a time: a
   `securesend_allow_guests` setting, an enable flag per kiosk, and very nearly a