This makes it easier for people to try out koushin.

Don't issue a CardDAV query with any filter, Google won't return any
result.

For now it can only list events for the current month.

References: https://todo.sr.ht/~sircmpwn/koushin/60

A new minimal CardDAV plugin is introduced. It injects a list of e-mail
addresses in the e-mail compose view.

References: https://todo.sr.ht/~sircmpwn/koushin/7

It's too slow on servers where indexes aren't enabled.

References: https://todo.sr.ht/~sircmpwn/koushin/23

This fixes an OpenBSD issue.

Closes: https://todo.sr.ht/~sircmpwn/koushin/50

Closes: https://todo.sr.ht/~sircmpwn/koushin/59

Add a settings page where the user can change the number of messages
displayed per page.

References: https://todo.sr.ht/~sircmpwn/koushin/5

References: https://todo.sr.ht/~sircmpwn/koushin/16

Note that attachments will be lost. This is a TODO.