Changes:
1. rename the sandstorm "manage" permission to "edit"
(old permission names: view, add, manage;
new permission names: view, add, edit).
Rationale: "edit" best describes this permission's current powers, to users and to operators.
If we ever added more manager-type features we'd want that to be a new permission,
not a rename of the existing one (which would change the powers of existing users).
2. rename the sandstorm roles for consistency with permissions
(old role names: viewer, editor, manager;
new role names: viewer, adder, editor)
Rationale: it's needed to avoid confusion.
3. add a new option: --allow=view|add|edit|sandstorm (default: add).
'sandstorm' sets permissions according to the X-Sandstorm-Permissions header.
Drop the --capabilities and --capabilities-header options.
Rationale: it's simpler and more intuitive.
4. replace "capability" with "permission" in ui/docs/code.
Rationale: consistent with the above, more familiar.
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