Run modernize

See https://pkg.go.dev/golang.org/x/tools/gopls/internal/analysis/modernize

It's mostly interface{} -> any, but also two quite useful applications
of slices.Contains.

Refactor config loading and server initialization

As is, LoadConfig() does some things that are not strictly related to
the configuration, such as parsing command line arguments. This has led
to a proliferation of different ways to load the config based on various
needs and also prevents tools that need a config but are not services to
use custom command line arguments.

This commit aims to decouple config loading from everything else and
do nothing but loading the config files.

On a high level, this commit:
- renames server.NewServer() to server.New()
- moves config.Debug and config.Addr into the server package
- moves crypto.InitCrypto() call into server.New()
- moves command line parsing into server.New(), using passed-in values
  rather than os.Args

The only changes required for services would be changing

   cfg := config.LoadConfig(":5100")
   server := server.NewServer("meta.sr.ht", cfg)

to

   cfg := config.LoadConfig()
   server := server.New("meta.sr.ht", ":5100", cfg, os.Args)

All other tools will be switched to just LoadConfig() and, optionally, a
call to crypto.InitCrypto(). I managed to completely remove some global
state (addr) and at least make the rest private, so that users are
forced to use the designated functions.

The config module gained support for custom FS implementation, mainly
for testing.

server/email: print correct var on type assertion failure

server/email: use go-message for address parsing

No need to additionally depend on net/mail

server/email: don't panic in EmailRecover

A panic in here is not great, because this function is supposed to
be the panic handler. Let's just log the error instead.

server/email: log missing mail::error-to, treat empty string as unset

Log a message when mail::error-to is unset, just in case the admin
forgot to set it. Don't try to parse an email address if error-to
is set to the empty string (the default value in the default config
file).

server/email: simplify and cleanup

- Only print the error once
- Remove unnecessary var
- Use debug.Stack instead of hand-rolled logic
- Ignore json.Marshal errors in a simpler way

go fmt

server/email: handle errors out of GQL context

server/email: include variables

go fmt

Remove %e formatting verbs

%e is not valid for formatting error values:

> %e	scientific notation, e.g. -1.234456e+78

Instead, use %w when using fmt.Errorf (for error wrapping) and
%v when using log.Printf (%w is not valid in that context).

Configure server to manage mail queue

This also sets up the recovery function for webhooks, so we get emailed
when we panic during a webhook query.

Sign outgoing emails

server: incorporate queue management

Add email work queue

Import GQL server interfaces from gql.sr.ht