Add ability to select TranasctionImportType to FileImport page

[bmbc-dev-detec]

Once the Transaction Import work is complete (#229) update the File Imports UI so that the user can select a 'Type' of import.
This provides 2 things:

1. The ability to import transactions ad-hoc (which is as it is right now)
2. The ability to manually upload a list of transactions for a specific transaction import type (as long as they're in the generic format that this process can work with)

[bmbc-dev-detec]

Would be good to review with @zoecrosby - particularly around the requirement of a TransactionImport to have a 'type' and therefore the requirement for File Imports to also have a type.

Perhaps we should have 'system' types - such as 'Manual Import' and instead of letting the user select a type? (I'm thinking that letting the user select which type may be useless, as the data for each type would need cleansing into the generic format, and who is going to do that? - they'd just run the source file through the import integration again if they wanted to upload data, say, after a reversal (which is still to be developed...))