4088: Add usage docs to filterable.js
4088: use filterable.js to filter on client side

Implemented client-side filtering via filterable.js:
* Designated filename rows as "filterable"
* Designated filename pattern input field as "filterable-control"
* Added "Select all" and "Unselect all" buttons with matching
  select_all_files() and unselect_all_files() actions
* Limit selection actions (compare, move, copy etc) to visible page
  elements only.
* Added integration tests "Filtering collection files by regexp" and
  "Creating collection from list of filtered files".
4088: code review feedback

* Modified styling for "Select all" and "Unselect all"
* "filename filter" -> "filename regex"
* Changed refutations to assertions to avoid running afoul of AJAX race
  conditions
* Refactored checkbox assertions into a helper
Merge branch '4088-collection-show-files-filter'

Fixes #4088.