Clarify if we are expecting a username or path

Problem/Motivation

The module currently shows a prefix of `https://www.drupal.org/u/` which implies that the user has to enter the rest of the URL path. However, the logic assumes that the user would enter the actual username. This is fine in most cases but when a user has a space in their username, then this assumption breaks. The username might be `First Name Last Name` but the URL would be `first-name-last-name` or something to that effect.

If the user enters the URL path, then the user lookup fails because we search on the username.

Steps to reproduce

Described above

Proposed resolution

We have to decide which one to support (or can we support both?) I am inclined to support usernames and if we go that route, we should just drop the prefix as it leads to this confusion. We can also consider making this configurable but that's a larger task and we could consider it in a future issue.

Remaining tasks

- Decide the fix
- Implement the fix

User interface changes

- If we go with keeping support for username, then the prefix should be removed.

API changes

- If we go with the approach of supporting the URL path, then the API for certain methods may behave differently.

Data model changes

None