Update strings files before CKAN 2.0 call for translations

Pull the latest strings from the CKAN 1.8 resource on Transifex into the
po files (some new translations have been added to 1.8 on Transifex,
since 1.8 was released). I've checked these using check-po-files and
they seem okay.

I did not commit the zh_CN (Chinese) or dv (Divehi) strings files as
these currently have 0% translations for CKAN 2.0, maybe they'll be
added before the release.

Update the ckan.pot file with the strings from the source files on the
release-v2.0 branch. I've pushed this updated pot file to Transifex as
the resource CKAN 2.0.

Update the resource slug in .tx/config.

Recompile the mo files from the updated po files.
Add British English translation from Transifex

For now I've added British English to the filtered-out languages, until
we can do the work to get it displaying properly in the Language:
drop-down.

Conflicts:
	ckan/config/deployment.ini_tmpl
[i18n] Pull translations from Transifex

Pulled the latest translations from Transifex with `tx pull`, checked
them with `check_po_files.py`, compiled the mo files with
`compile_messages`.