refactor teamIds to teamId on Provider schema

[mattkrick]

With some integrations having clickless oauth, there's no value in the original plan of having multiple teams share an oauth token (unless a provider nullifies your old token? we should check with github...)

[mattkrick]

github does not nullify old access tokens, which is great.
currently, we create 1 Provider per TeamMember, so no hard migrations needed.
the main benefit is the speed up for the client, since we don't need to contact parabol to see if they have a provider on another team that can be used.
the benefit for me is that i don't have to iterate over an array of 1 all the time.