Warn when --fake marks multiple migrations as run

[AnayGarodia]

Closes #1255

piccolo migrations forwards --fake marks pending migrations as run without applying them. When several migrations are pending, running it with --fake silently fakes all of them, which is an easy way to lose track of which schema changes were actually applied.

As agreed in #1255, this adds a warning when --fake will mark more than one migration as run, printed right after the "Running N migrations" line and before they are faked. A single faked migration is unchanged.

The regression test (test_forwards_fake_multiple_warns) fakes the music app's migrations with fake=True and asserts the warning is printed; it fails on master and passes with this change. The rest of the forwards/backwards suite still passes.

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[dantownsend]

Thanks for this. I made a tweak - so we use input to ask the user if they want to proceed.