4088: filter collection file view by regex

Added a "Filter" field for supplying a regular expression to filter
filenames. Updated integration tests.
4088: code review feedback

* Filter files by regex before .take(10000)
* Added "regular expression" placeholder in the filter input box
* Issue a Rails "alert" div if the regex could not be parsed
* Test that we're actually viewing a collection when no matches are
  found (and not a fiddlesticks page or something)
4088: fix alignment for right-floating input field
4088: filter only on files

Only apply the regex filter to files; continue to process subdirectories
as normal in order to keep the appropriate filename hierarchy.
Merge branch '4088-collection-show-files-filter'

Refs #4088.