this should be enough "merging" of lookup&search:
* partial usernames are searched as part of notes from the widget, and
as part of known usernames in "search users"
* tags are searched as part of notes from the widget and the "search
notes" page
* full usernames always navigate to the profile page of that
user (which will fetch the profile if possible)
as an extra nicety, if "search notes" is disabled, the search widget
handles hashtags like the lookup function does
