Allowing the user to view deleted messages creates all sorts of race
conditions. The most devious race condition is pv.source can be set to a
nil while another PartViewer is still running a goroutine in
attemptCopy.

Here is a trace when this happens.

goroutine 76 [running]: io.copyBuffer(0x7f8ad02641d0, 0xc00040f590, 0x0,
0x0, 0xc0007cc000, 0x8000, 0x8000, 0x0, 0x0, 0x8b3d60)
/usr/lib/go/src/io/io.go:402 +0x101 io.Copy(...)
/usr/lib/go/src/io/io.go:364
git.sr.ht/~sircmpwn/aerc/widgets.(*PartViewer).attemptCopy.func4(0xc00017efd0,
0xc0004da7c0) /home/keur/repos/aerc/widgets/msgviewer.go:576 +0x611
created by git.sr.ht/~sircmpwn/aerc/widgets.(*PartViewer).attemptCopy
/home/keur/repos/aerc/widgets/msgviewer.go:544 +0x144

We could add a guard in store.FetchBodyPart to only call the callback
when msg.Part.Reader != nil, but we still get a hanging pager. Therefore
it seems more reasonable to disable this completely.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

+ Changes NewComposer to return error.
+ Add lib to handle templates using "text/template".
+ Add -T option to following commands
    - compose.
    - reply
    - forward
+ Quoted replies using templates.
+ Forwards as body using templates
+ Default templates are installed similar to filters.
+ Templates Config in aerc.conf.
    - Required templates are parsed while loading config.
+ Add aerc-templates.7 manual for using template data.

`go-sasl` upstream added support [1] for the obsolete [2] AUTH LOGIN
method which enables aerc to send email via servers which remain common
in the wild.

Fixes ~sircmpwn/aerc2#263

[1] https://github.com/emersion/go-sasl/commit/61afe53d
[2] https://datatracker.ietf.org/doc/draft-murchison-sasl-login/

A pager is spawned every time an email is viewed but not killed off when
quitting the msgviewer, thus leading to process leakage. This patch
fixes this by adding a Close method to the msgview widget, which is
called in the close command.

Signed-off-by: Kevin Kuehler <keur@xcf.berkeley.edu>

As suggested in #268, this adds a behaviour setting for
deleting message from message viewer.

This patch ensures the following commands join their arguments with
spaces to make it easier to interact with:

- cf
- mkdir
- cd
- attach
- detach
- ct
- copy
- move
- save

There is a command and config option. The criteria are a list of the
sort criterion and each can be individually reversed.

This only includes support for sorting in the maildir backend currently.
The other backends are not supported in this patch.

This adds the ability for per-account signatures in the accounts.conf
config file. The signature is added to emails in the editor at the
bottom of the email. This includes when forwarding, replying to, and
composing emails.

There are two config options: signature-file and signature-cmd. The
former allows a signature to be read from a file and the latter allows
an arbitrary command to be executed to return the signature.

The config options have been documented in aerc-config