This will stop us repeatedly forgetting to add `novalidate` and
`autocomplete='off'` to our forms (which is how most of them are set
up).
It uses sensible defaults, based on how we most-commonly configure
forms:
- most of our forms are `post`ed (but this can be overridden)
- `autocomplete` should only be enabled where it makes sense, otherwise
it’s more annoying than useful (but this can be overriden)
- we should never be using HTML5 form validation because our own error
styles and messages are better
We’re extracting this from being determined based on what the sender
name is to its own setting.
This commit will let users set it independently.
Until the explicitly set it, it will still be determined based on
whether their default sender name matches the default for the platform.