Set "sent" folder for Mutt

[tstearns]

As discussed on IRC, the default ~/sent folder causes problems for new users who get "read only filesystem" errors. This change puts the sent folder inside the maildir which is configured as ~/Mail.

[KellerFuchs]

Hi, @tstearns ! Thanks for sending that pull request.

I just rolled back @lrvick's merge because:

1. he didn't do a signed merge, resulting in new users not getting dotfiles at all;
2. this is a breaking change (users need to be made aware of it and move their sent mails).

Could you reopen that pull request and say on which server you encountered the issue?

[tstearns]

Observed on da1.hashbang.sh and ny1.hashbang.sh from the default .tmux_bootstrap which runs mutt inside firejail which causes the following error when trying to send email, since mutt's default record folder is set to $HOME/sent:
$HOME/sent: Read-only file system (errno = 30)

[KellerFuchs]

OK, I can reproduce the issue.