Manage templates, categories and comments over AtomPub (#209)

* Manage templates, categories and comments over AtomPub

Add three AtomPub collections to each weblog's workspace:

- Templates (/{handle}/templates): create, read, update and delete a
  custom theme's templates, with the standard rendition as atom:content
  and the mobile rendition as roller:rendition. Weblog administrators
  only, and only while the weblog uses a custom theme and the site
  allows custom themes. An empty mobile rendition removes it.
- Categories (/{handle}/categories): create, read, update and delete.
  A category in use is removed only with ?moveTo={id}. The entries
  collection lists categories inline and also references a new RFC 5023
  category document at /{handle}/categories.atomcat.
- Comments (/{handle}/comments): moderation only. List (filter by status
  or entry), change roller:status, delete. thr:in-reply-to (RFC 4685)
  links to the weblog entry.

Move the template rules from the Templates and TemplateEdit pages into
WeblogTemplateEditor so the UI and AtomPub enforce the same rules, and
add WeblogManager.removeTemplateRendition() for mobile renditions.

* Address PR #209 review findings

- Refuse to delete a weblog's last category (409), as the Categories
  page does.
- Make isWeblogCategoryInUse() null-safe for weblogs without a default
  category.
- Test that deleting the default category with moveTo hands the default
  over, and that the servlet serves the category document.

* Apply the category editor's field rules to AtomPub

- roller:image must be an http or https URL, as on the Categories page,
  which now uses the same check. The category wrapper gives templates
  only such URLs.
- Category names and descriptions, and template names, links and
  descriptions, are limited to 255 characters, as in the editors.
- Test that AtomPub tags use the shared tag character rules.

* Add an AtomPub reference guide (#211)

* Add an AtomPub reference guide

docs/roller-atompub-guide.adoc documents Roller's whole AtomPub
interface: enabling and limits, BASIC and OAuth authentication, the
service document, and the entries, media, category, template and comment
resources (URIs, permissions, elements, status codes), with a curl
session and an error table. It is built to PDF and shipped with the
other guides.

- docs/atompub/examples: XML examples included by the guide. Response
  examples are generated from AtomWriter by AtomPubDocExamplesTest, which
  fails when they no longer match (-Datompub.docs.update=true rewrites
  them). Request examples are hand-written and must parse.
- docs/atompub/roller-atompub.rnc: RELAX NG grammar for the roller:* and
  thr:in-reply-to elements; the test validates every example against it
  and against the RFC 4287/5023 grammars.
- Move the Jing helper into RelaxNgValidator for both schema tests.

* Document the category image and length rules in the AtomPub guide