`start_date` should be an optional field on all connectors

## Tell us about the problem you're trying to solve
Asking users for more inputs reduces activation rates. Whenever possible, we should use sane defaults on behalf of the user. Most API connectors declare a `start_date`. But there is no reason for this to be a required input (currently, it is required most of the time).

## Describe the solution you’d like
`start_date` should not be a required input on any API connector. When `start_date` is not input, we could either:
1. use a default `start_date`
2. load all the data in the user's account (I think this is the better option as that's probably the most intuitive option for the user)

## Suggested Implementation
I suggest that we first validate this is a useful idea by implementing it for 3-5 connectors which are frequently used but have a higher-than-desired activation-failure rate (presumably due to their setup being too hard).

Once we roll out that change and observe any discernible improvement in setup rates, we should sequence the implementation of this issue into batches of 5-10 connectors at a time, ordered by usage/importance of the connector. E.g: first do the top 10 most used connectors, then the next 10, etc.. . This allows us to incrementally implement this improvement and get the benefits quickly. And if we need to shift gears halfway through, we can stop without losing all our progress.