Drop deprecated admin/forum/index method.
Separate `add_raw_data` calls from Jade markup.
Minor/style changes:

- Drop field filtering on some Mongo queries.
- Small code reorganization at section/index.
- Small code reorganization at section/update.
- Rename `active_section` to `current_section` at section/index.
- Rename `other_sections` to `allowed_parents` at section/index.
- Rename `collectSectionsTree` to `buildSectionsTree`.
- Rename `updateSection` to just `section` at section/update.
- Rename `copySection` to `copySettingsFrom` at section/form.
- Rename `targetSection` to `selectedSourceSection` at section/form
- Rename `payload` to `request` on RPC calls.
Add comments on section/update server method.
Refactor sections tree render mixin at section/index page.
Fix lib/to_tree.js on sections with `parent` === undefined

This applies to demo category created by the migration.
Refactor section/destroy server method.
Drop `allowHoles` option of `forum_usergroup` store.
Refactor section/form block.
Add support of subtree-only updateInherited for `forum_usergroup` store.
Move `updateInheritedSectionData` to Section model hooks.