Fixes:

  - Message footer/header files of domain/site default are put in `mail_tt2` subdirectories while those of list config are not.
    Fixed by put all of them in consistent location without intermediate directories (See also Notes below).
  - Entries for header/footer files in `edit_list.conf` were ignored.

Changes:

  - File names themselves of header/footer were changed:
    `message_header`, `message_header.mime` etc.
    This is due to concideration about conflict between files and directories with domain names (See also Notes below).
  - Header/footer files with `.mime` extension:
    Previously, files with this extension were always preferred, then files without extension were used.  This behavior has not been documented.
    Now files with extension will be searched only in case `footer_type` is `mime`.  Files without extension will be searched in both cases.

Notes:

  - Files with older names/paths in list/domain/site config directories will be copied to new location during upgrading process.
  - Template files with old names in families (`message.header.tt2` etc.) will be used as before and will be copied to new location during instantiation. If files with new names (`message_header.tt2` etc.) will be preferred, if they will be added.

Known bug:

  - There are no web UI to edit global footer (`message_global_footer`).