Implement activity buffer/cache

[ahal]

I want to let users set an activity buffer (cache size) which we will immediately fill with activities. When an activity is viewed it gets removed from the buffer (when there are 10 empty spaces we re-fill it).

This way:
a) The user doesn't need network connectivity
b) The user never has to wait for the next set of activities to load

This will be slightly tricky since we will need to think about filtering and stuff like that. Maybe the buffer could be used as a backup in case there is no internet connection.

I'm going to say this will likely not be targeted for the demo, but it's something I'd like to do long term.