15610: Avoids loading the file list on big collections, offers manual loading.

After the previous performance enhancements, 75% of the time spent to show
the collection's files goes to the WebDAV request + parsing, so to avoid
inadvertently freezing the UI, when the file_count field passes a predefined
value (now 40k files), the user gets the option to manually load the file
listing by clicking on a button.

Arvados-DCO-1.1-Signed-off-by: Lucas Di Pentima <lucas@di-pentima.com.ar>