This refactors the TodosFinder finder according to the new code reuse
rules, as enforced by the CodeReuse cops. I also changed some of the
methods to use regular if statements, instead of assignments and/or
early returns. This results in a more natural flow when reading the
code, and it makes it harder to accidentally return the wrong result.

This reverts commit 8717c7da.

This reverts commit 4d9a3f42, reversing
changes made to ecf9c145.

* Group filtering now includes also issues/MRs from
subgroups/subprojects
* fixed due_date
* Also DRYed todo controller specs

This was causing todo priority sorting to fail.