Auto-create Sent and Drafts default folders

[stephdl]

Dovecot only auto-created Junk and Trash in the inbox namespace. Sent and Drafts were never created by the server, so they appeared only after the user sent a first message. When an IMAP client is configured before those folders exist, it can create duplicates (e.g. Italian Outlook creates Posta inviata instead of Sent) or fail to copy sent messages.

This declares Sent and Drafts as auto-created and subscribed mailboxes with their special_use flags (RFC 6154), so they exist at first mailbox access regardless of the client or webmail (Roundcube, WebTop, SOGo, IMAP clients).

Changes, mirroring the existing DOVECOT_TRASH_FOLDER pattern:

• local.conf: two new mailbox blocks in the inbox namespace, auto = subscribe, special_use = \Sent / \Drafts.
• reload-config: tmpl_sent_folder / tmpl_drafts_folder template variables (defaults Sent / Drafts).
• build-images.sh: DOVECOT_SENT_FOLDER / DOVECOT_DRAFTS_FOLDER build-time defaults.
• README.md: documented both variables.

Existing users get the folders at next mailbox access, no migration needed.

Ref: NethServer/dev#8104

This supersedes NethServer/ns8-roundcubemail#116, which only covered Roundcube at webmail login.