Add the ability to mass delete files from the yara rule UI (checkbox style)

[danny248]

[dcuellar322]

@danny248 Done. https://github.com/InQuest/ThreatKB/pull/384

[dspruell-i01]

Not quite sure this is implemented; we don't seem to see a way to select multiple files and mass delete from the file list.

Screenshot shows no checkbox for multi-select. That said, this is not a critical issue at this time and we'll close it out, just adding a note.