merge: more Context elimination

 * add EmailList for tagged Emails
 * refactor edit_or_view_one_message
 * mutt_ev_message: factor out Context
 * bounce_message: factor out Context
 * mutt_check_traditional_pgp: factor out Context
 * crypt_extract_keys_from_messages: factor out Context
 * mutt_link_threads: factor out Context
 * mutt_label_message: factor out Context
 * pipe_msg: add Mailbox param
 * pipe_message: factor out Context
 * mutt_save_message: factor out Context
 * mutt_change_flag: factor out Context
 * ci_send_message: factor Context out of mutt_make_string()
 * ci_send_message: factor out Context
 * ci_send_message: factor Context out of dependents
 * mutt_mailbox_check: factor out Context
 * mx_mbox_sync
 * add mx_path_resolve()
 * mx_mbox_open: drop path param
 * factor out PostContext
 * tidy code
 * tidy Email variables
 * move parse_mailboxes()