New `SessionNewAction` DSO, 1 of 2.

New DSO calls (new) `UserRepository#authenticate` method with passed-in
user (name) and password strings. Note that calling `run!` on this DSO
will raise an error if and only if parameter validation fails; you'll
have to look inside the returned `StoreResult` instance for more detail.

RSpec: 914 examples, 0 failures (but see Issue #87)
RuboCop: 222 files inspected, no offenses detected