Create a single point in which the basic authentication header is processed.

Previously, basic auth header was processed from two different servlet
filters in a single filter chain.

In case the 1st filter (ApiTokenFilter) manages to authenticate the
request, the 2nd filter (BasicProcessingFilter) tries to avoid
interpreting the API token as the password and failing authentication
(see BasicProcessingFilter.authenticationIsRequired), but the check
feels rather fragile.

Although I did eventually discover that the original problem (ZD-19640)
was not caused by this, I've already implemented & tested this change,
and this feels like a good work to be wasted, so I'm pushing this in
anyway.

Refrence: ZD-19640