Measure session (data) “size” and warn about large sessions on `SessionViewdata` and `SessionData`

[mrpg]

This is also we added this to the docs. Large sessions can, when data is requested by the browser to be viewed or just downloaded, slow down the server. Need warning about this.

[mrpg]

Thanks to 6a3f210 (#275), downloads are fast now. Nonetheless, I don’t like the “waiting period” when opening SessionViewdata.