Improve Join/Connect button language on activity pages

[madprime]

When on an activity page for an activity a member hasn't joined, there's a button inviting them to join it.

1. Connect vs join: To join projects right now, it seems the activity pages say "Connect" for any OAuth2 project ("Connect project") regardless of whether it functions as a data source. This probably makes sense in a lot of cases (e.g. "connecting" fitbit intraday) but might be confusing at other times (e.g. any OAuth2 project that isn't primarily for "adding data"). We should look into whether there's a better logic on whether to say "Connect" instead of "Join".

2. Study vs. project vs. activity: It seems that all activities say "Connect project" or "Join project". Since we're moving to say "activity" it seems that "project" should be retired here. Also, for any activity that is an official study, maybe it should say "study" instead?