cache files

[gagata]

we don't have to load all the data from FB all the time.

1. store locally all the information about files
2. add column to the group view with last update time
3. load only new files (last update till now)

Problems:

• still we need to check availability of stored files
• sometimes we need to refresh the data (for example - sb might delete the post, so we should not present it to user)