Collaborator search update / route security fixes

[mpsonntag]

This pull request introduces the following changes:

RepositorySettings: Collaborator search updates

• fixed missing dropdown when login name was entered perfectly.
• fixed dropdown toggle: toggle is now reset when a new search is started.
• search has now a minimum length of 3 characters to reduce auth server requests.

Route security fixes (#110):

• Manually entering the route for the Request DOI page of a repository without being the owner leads to a redirect to the "Not found" page.
• Manually entering the route for the Settings page of a repository without the proper permissions leads to a redirect to the "Not found" page.
• Manually trying to access a private repository without proper permissions or manually entering the route for a non existing user or repository will now display a specific "not found" message without incapacitating the page, yay (see #108 for details).

Closes #98, #110.