annotateitpermissions: set user correctly for new annotations

The AnnotateItPermissions plugin previously partially inherited the Permissions
plugin's `addFieldsToAnnotation` method, causing the 'user' field on new
annotations to be set, incorrectly, to the entire auth token rather than the
string user ID of the authenticated user.

The result of this was that the authenticated user didn't have 'admin'
permissions on brand new (unsaved) annotations, and so the "Allow anyone to
{view,edit} this annotation" checkboxes didn't appear in the UI.

This commit adds a test and a fix for this behaviour.
tools/serve: make consistent with dummy store

The default configuration of the Annotator store in test mode uses a user of
'alice' and a consumer of 'mockconsumer':

    https://github.com/okfn/annotator-store/blob/master/run.py#L48-L54

This commit makes tools/serve use a user and consumer consistent with this by
default.