* hledger-web extra tests

Additional, manual tests for hledger-web.
These should be moved to the automated test suite when possible
(Hledger/Web/Test.hs, `stack test hledger-web`).

** hledger-web -f/dev/null, a, submit a transaction: should succeed
Adding to a single file, with no file field showing in the add form, should work. (#1932)