Introduce FileActivity type "CONTENT_CHANGE"

[tobous]

trafficstars

Currently, Saros/E sends a lot of binary content updates through file creation activities for existing files.

Saros/I is currently not set up to handle this as it throws an exception if Saros tries to create an already existing file.

In general, using file creation activities for content updates seems strange to me. Instead, we should introduce a new file activity type CONTENT_CHANGE for such occasions.