Fallback to existing mention label if provider does not return a label

[gehrisandro]

Description

This PR changes the mention rendering to fallback to the stored label in the database according to the discussion here: #18984

In addition the label gets persisted to the database even if a MentionProvider is available (otherwise there is nothing to fall back to).

Visual changes

Before (when user is not available anymore):

After:

When the provider returns a label (in my case, the user still exists), there is no visual change.

Functional changes

• Code style has been fixed by running the composer cs command.
• Changes have been tested to not break existing functionality.
• Documentation is up-to-date.(no change required)