Rename display name to "Meta Photo Nav" and plugin ID to meta_photo_nav
(constants META_PHOTO_NAV_*, maintain class meta_photo_nav_maintain).

Add the release pipeline used by the other Piwigo plugins:
- maintain.class.php (no-op PluginMaintain subclass for the plugin manager)
- build-plugin.sh repacks the plugin under its clean ID folder meta_photo_nav/
- .gitlab-ci.yml (shell executor) builds the package on tag, uploads it to
  the generic package registry, and creates the release via the Releases API.