This fixes the bug where if you have three placeholders:
> ((one)) ((two)) ((three))
The first one you are asked to fill in is `((three))` (ie
`template.placeholders[-1]`).
This reintroduces a bug where if you use the ‘Use my phone number’ link
you skip straight to filling in `((two))` and can never proceed because
you’re never given the chance to fill in `((one))`. This commit also
fixes that bug.