Uploads can fail partway through

[calpaterson]

Description

We only peek at the first 100 lines of the file to infer the types of columns. If the type changes (say 1.1 in an INTEGER column) after line 100 we fail to import at all and the user sees a 500.

We need to infer the types from the whole file and probably present the user with options via a new UI

Steps to reproduce

1. Paste a file where the format of a column changes after 100 lines
2. Then click "Create!"

Expected result

Sees some further UI to allow them to specify the types correctly (rule out incorrect options)

Actual result

500 error

Additional details