Log Message:
gitea: Various fixes.

Add a target to generate the frontend artefacts that are required at
runtime, but do not build them by default as they require nodejs/npm which
will download node modules during the build.  Instead use a pre-built copy
stored in LOCAL_PORTS.  The web interface should now look correct.

Use the supplied Makefile to simplify some of the targets.

Remove unnecessary MAKE_DIRS (shadows an OWN_DIRS_PERMS), and fix
permissions on some of the installed files.

Based on joyent/pkgsrc#263 from GitHub user teutat3s.