Improve seeds

[ukutaht]

Changes

Improves the script that generates seeds. The improvements compared to master are:

• Imported and native visitor numbers are roughly similar now (~10x difference on master)
• No empty page in "Top Pages"
• Events are generated as part of a session with a stable user id and steadily increasing timestamp
  • Visit duration looks more realistic
  • Views per visit looks more realistic
  • Time on page looks more realistic
• Configures the logged_in prop during seeds so you don't have to do it manually
• Adds some utm_mediums so that channels report is more interesting
• Potentially some more stuff that doesn't cross my mind at the moment