Move interface translations out of content packs

[benjaoming]

Summary

We call them "content packs" for a reason.

Interface translations tie directly to the software release and should as such be released with the software and not sitting separately in these rather huge .zip files for content packs, which we'd rather avoid having to update.

Architecture of new translation:

• Locale files such as django.po/mo are stored conventionally. We want to have everything stored in the project home directory, kalite/project, as the messages are collected throughout all the apps in kalite/. This means moving the current output directory kalite/locale to kalite/project/locale.
• New make target pull-msgids fetches messages from CrowdIn
• New make target push-msgids pushes source msgid's to CrowdIn with the right name (we have this versioned naming scheme, let's just stick to that).
• MANIFEST.in should ignore *.po but ensure we have *.mo

I order to fix this, we need to:

• Remove old functionality: Stop unpacking .po files from content packs
• Remove old functionality: Remove ~/.kalite/locale from LOCALE_PATHS, it shouldn't be necessary anymore
• Ensure that new path kalite/project/locale works
• Add configuration for crowdin-client to point to our correct CrowdIn resources
• Create make targets pull-msgids and push-msgids

I'm targeting this for 0.17, because I see that we can fix this:

1. without rebuilding content packs, just leaving the content packs as they are - 0.17.4+ won't use .po resources from content packs, and everything <0.17.3 will.
2. being backwards compatble
3. avoid building content packs again once the 40 strings in Missing translations for webpack bundles/*.js files #5517 are translated

[benjaoming]

There was an unfortunate and confusing error in the description, sorry about this @mrpau-eugene

Ensure that new path ~/.kalite/project/locale works

Becomes:

Ensure that new path kalite/project/locale works