Improve activity feed loading

[cgrisdale]

Currently the activity feed shows 17 events when you land on the page and clicking on "Load More" loads an additional 17 events. The activity feed can get filled-up very quickly, especially when many revisions have been submitted as each revised field shows up as two separate events (this will be improved by #512 ), making it difficult to get a view of activity over a longer time-span.

I think it would be beneficial to either have options to load 25/50/100 events at a time or have auto-loading when you scroll to the bottom of the list.