merge: factor out Context

 * notmuch: factor out Context
 * factor out Context from msg_padding_size()
 * factor out Context from tags_commit()
 * factor out Context from tags_edit()
 * factor out Context from mx_tags_is_supported()
 * light tidying
 * move Context flags to Mailbox
 * factor out Context from mx_msg_commit()
 * factor out Context from mutt_newsgroup_catchup()
 * factor out Context from mutt_newsgroup_uncatchup()
 * factor out Context from find_subject()
 * factor out Context from make_subj_hash()
 * factor out Context from check_subjects()
 * factor out Context from mbox_open_append()
 * factor out Context from mutt_messages_in_thread()
 * factor out Context from link_threads()
 * factor out Context from mx_toggle_write()
 * factor out Context from mutt_score_message()
 * factor out Context from maildir_update_flags()
 * factor out Context from mutt_check_rescore()
 * factor out Context from mutt_prepare_template()
 * factor out Context from append_message()
 * factor out Context from mutt_append_message()
 * factor out Context from mutt_set_flag_update()
 * factor out Context from mutt_copy_message_ctx()
 * factor out Context from mutt_save_message_ctx()
 * factor out Context from mutt_make_message_attach()
 * factor out Context from trash_append()
 * factor out Context from edit_or_view_one_message()