Closes #40645

This was caused by the `@projects` value being empty when the current
user does not have any personal projects.

This allows users to quickly switch between all projects they have
access to & there own namespace projects. These tabs also keep
the same filtering/search options selected so the user can quickly
switch between the two different contexts.

Closes #29045

Closes #36459

Closes #36317.

In various places we check if the same relation would return projects.
This is done using "any?" which will run a COUNT query with any
LIMIT/OFFSET values still applied.

To work around all this we introduce 2 helper methods that take care of
doing the right thing. This leads to the produced queries being simpler
and fewer queries being executed.

`breadcrumb_title` is then overriden when needed