feat: journal:fix-headings and the settings API

- classes.FixStoredHeadings converts stored H1 in post content (every
  locale) and category pillar text, in one transaction with a RecordSaved
  and a site cache purge per changed record
- journal:fix-headings, a dry run unless --apply, refuses with exit code 1
  while the H1 setting is off
- GET/PUT /_journal/api/v1/settings and POST /fix-headings {dry_run} for
  golem15.journal.manage_settings; 409 with the refusal while the setting
  is off

plugin.go

@@ -122,6 +122,7 @@ func (p *Plugin) Commands() []bonfire.Command {
 		console.ExportPosts(p.app),
 		console.ImportPosts(p.app),
 		console.Reindex(p.app),
+		console.FixHeadings(p.app),
 	}
 }