added modified Seafile PKGBUILDs from AUR

With these PKGBUILDs it's possible to compile and run Seafile on arkOS.

I made the following modifications:
- removed 'hicolor-icon-theme' and 'gtk-update-icon-cache' dependencies from seafile-shared - they are only needed for seafile-client (maybe it's possible to remove seafile-shared completely in the future and move everything needed to the seafile-server PKGBUILD)
- removed 'python2-djblets' and 'gunicorn-python2' dependencies from seafile-server and modified seafile-server.install to install them using virtualenv instead

The dependencies
libzdb python2-flup python2-simplejson python2-webpy
need to be added from the ArchLinuxARM community repository.