IS_SELF_HOSTED should default to true

[arthurwolf]

as per JCT-122 and comments in wireapp/wire-server#2515

IS_SELF_HOSTED should default to true.

for team-settings I made a PR, but here there are multiple mentions of it, and it's not as straightforward, so I'm creating an issue for either somebody else to make the change, or for somebody to give me directions on making the change.

thanks