[JIRA-81] Web UI Pagination

add pagination to in memory files' view in web UI, only show 20 files a page. 

without pagination, when there are too many files in memory, it will take a long time to render the 'in memory files' page.
It would be simpler if we move away from pages and into offset/length

offset=0&length=20 (same as page=1)
offset=20&length=40 (same as page=2).

This also gives the user more control