Closes: https://todo.sr.ht/~sircmpwn/koushin/33

Closes: https://todo.sr.ht/~sircmpwn/koushin/64

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

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

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

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

This allows registered plugins to execute code when loaded. This will
also allow the Lua support code to be a plugin.

Closes: https://todo.sr.ht/~sircmpwn/koushin/54

And mark original message as answered.

Closes: https://todo.sr.ht/~sircmpwn/koushin/15

This only reloads templates for now. In the future it'll also reload
Lua plugins.

Take a *Context instead of a echo.Context. This saves a type assertion
in each handler.

This allows Go plugins to easily inject global data.

RenderData will be used for an interface.