Support named spring URLs

Add a controller that handles named URLs for springs, such as

  https://my.tree.net/s/rekado/news

This is the first step to make spring pages that contain public posts
accessible for people without an account on Libertree.
remove embedder libs
update libertree-model
use libertree-model with relaxed nokogiri dependency
widen comment textarea on excerpt view
remove unused style declaration
Merge branch 'spring-routes'
remove reference to narrow layout
new style for pool edit page
wrap form elemens in div.field, label
use new style on pools index page
wrap form elements
remove horizontal line between rivers and pools
center headings, add border
remove indentation from sidebar items
remove icons from sidebar, link headings
remove unused style declarations
no-more-posts should always be a div
only indent div.panel if it is a direct child of .main
width to max-width (works better on small screens)