Improve supported sieve extensions enabled by default using require

These are the dovecot pigeonhole sieve extensions included in dovecot by default that we don't currently support in custom sieve filters, and the version of pigeonhole which first included them. This list is from the Dovecot wiki.

Extension	Supported since
body	always
duplicate	v0.4.3+
enotify	v0.1.3+
environment	v0.4.0+
foreverypart	v0.4.14+
ihave	v0.2.4+
include	v0.4.0+
index	v0.4.7+
mime	v0.4.14+
extracttext	v0.4.14+
variables	always

We should definitely be including body and variables as they are enabled by default and supported in every version of dovecot pigeonhole. Body in particular is vital for many custom filters. For simplicity's sake I'm submitting a merge request for these 2 extensions straight away so it can hopefully become part of %3.2.9. Having sane defaults is especially important because the list of required extensions can't be updated later in the filter due to a limitation of sieve/pigeonhole: require commands can only be placed at top level at the beginning of the file (See #5124)

As for the others, currently the most recent extension we use is date with a release version of v0.1.12 from 2010. The most recent pigeonhole version needed to support all these extensions would be v0.4.14 which is from April 2016, over 6 years ago. Could we safely assume that all installations would have at least that version? Perhaps for these more specific extensions we should make no default inclusion and simply carry on letting admins enable them by installing a modified config into conf-custom? Some of these extensions also have security considerations such as include which allows including other sieve files.

What do you think @jnorell?