I18n cleanup

This patch cleans our i18n code in Katello.

a) app.pot and */app.po was renamed to katello.po[t] in the 868bb17e83d84ce2259f248c760eb3f3451710ac commit. If you encounter any issue merging upcoming translations from Zanata or Transiflex, revert the commit, merge and rename manually again.

b) src/locale/Makefile was introduced and documented:

```
$ cat locale/README 
How to work with locales
------------------------

1) Extract strings:

bundle exec rake gettext:find

2) Translate strings using Zanata or other tools

3) Generate mo files using GNU gettext (in SPEC):

pushd locale
  make
popd
```

There is additional pofilter target

    make check

which was moved from SPEC file here so you can check this regularly.

c) During build phase, MO files are generated using GNU gettext and Katello was reconfigured to read MO files instead of PO files. PO files are no longer distributed.

If possible check Zanata and Transiflex now with renamed files, I am not sure how to work with these tools, but I suppose we must somehow check-in renamed files.

I have tested the change, it works for UI. Unfortunately CLI I18N code is not working right now, I will create another patch which will fix this regression (for some reason we do not distribute locales anymore).