Fixes for docs, tickets, and refinement sessions

A round of fixes for things you told us about:

• Doc titles stay put. Renaming a page and then, say, adding an image could quietly put the old title back. Your new title now sticks.
• The doc editor scrolls properly. Only the writing and preview panes scroll now, so the toolbar and sidebars no longer slide out of view on long pages.
• Ticket templates follow the type. When creating a ticket, switching its type swaps in the new type's template, but only if you haven't written anything of your own yet. Anything you've typed is kept.
• Closed sessions are read-only. Once a planning poker session or retrospective is closed, it can still be viewed, but no more votes, cards, or changes can be added to it.