Piccolo-admin

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Fixed some bugs with optional fields in custom forms (thanks to Skelmis for
reporting this).

Improved the contribution docs (thanks to Skelmis for this).

Improved the MFA docs (thanks to sinisaos for this).

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Forms can now be grouped in the sidebar. This is useful when you have lots of
forms and want to keep things organised. Thanks to sinisaos for this.

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Added a missing JPEG file for an example form.

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Added more example forms.

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Piccolo Admin forms can now be used to download files, e.g. CSV and PDF files,
which are really useful for reporting purposes. Many thanks to sinisaos for
the help on this.

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Make the `MFA Setup` URL relative, so it works when Piccolo Admin isn't mounted
at the root.

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