Add preliminary filtering step before actual selection of authors

The extra step is currently transparent: all authors are kept. I will
then compute the set of selected authors in this filtering step,
and use it to reduce the list of authors before applying the selector
function.
Compare author selection flag explicitly with true

While a flag is set to true or false for each author in current version,
flags will soon be set only for selected authors, while no flag will be
found for authors not selected.
Use an object for the set of selected author flags

A value is set to true for each id of selected authors.
Extract function applyFilters() from anonymous callback
Add shortcut to publish all authors when no active filter is selected

The initial selection is now triggered by the "active-filter-list"
event published in collect-active-filter.js, which sets the full
set of properties expected in filter-author.js when the
"active-filter-selector" event is received.
Move definition of alwaysTrue() to utils.js for reuse
Replace flags to check whether an author is selected with a function

The function takes an author id as parameter, which makes it very close
to the check in a set of flags, with a difference: when all authors are
selected, the function is simply alwaysTrue(), there is no need to set
a flag to true for each and every author.