special-case full usernames is search #264

Created by: dakkar

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

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests