Cleanup: Move create_local_tm() to pootle.core.initdb

This way we:

* Consolidate code on pootle.core,
* Keep a cleaner code on the upgrade module,
* Keep all the DB initialization code in one place.
Return JSON when the error happened in an XHR context
Set maximum required Django version to 1.6

Code is not yet fully adapted to Django 1.6, mainly because of some third
party apps, some of them which are still in our codebase, e.g. djblets.
Expand makefile help with assets rebuild command
Adjust defaults to newer DDT versions
Add missing import
Cleanup: Use styleguide conventions on Goal fields declaration
Goals: Flush stats caches only in current translation project

Removing the cache in all the translation projects in the same project when
saving a translation was taking too much time.

Partially amends commit cc0ed4b4. Performance fixes in the next two commits.
Goals: Refactor out the code for getting the trail to a given path
Goals: More efficient cache flush for a given path

Now all the affected objects are retrieved before handling them. This avoids
retrieving the objects, for example the directories, several times thus
querying the database several times for the same object.
Docstring improvement: include example and rephrase
Cleanup: Only import if necessary
Whitespace cleanup
Removed unused import
New methods for getting stats

This incorporates commits (with necessary changes for our code base):
https://github.com/evernote/pootle/commit/f7bfd01d66b6c7e846be756a39318369d0884415
https://github.com/evernote/pootle/commit/7c89432a5a79db98176e7faf5e487a2164d650ff
https://github.com/evernote/pootle/commit/2d175ba08d4e82b0277708791cdff9ca21620d27
https://github.com/evernote/pootle/commit/76130849e2111c4c415e1d33cd274f53256124a8
https://github.com/evernote/pootle/commit/2de66822ed4a949b9bb3d9d9dab6c7988d2f40ea
https://github.com/evernote/pootle/commit/3591186540d2f049f5653910e78b0987dee9b9ef
https://github.com/evernote/pootle/commit/9e9031d8aec1309698491725e2d9dc7328eb8975
lastaction member stub
Implement get_name method override for the Directory object
Hide placeholders and table headings until the table is loaded

To indicate the table is loaded, remove 'js-not-loaded' class name from the stats table
Added JS for stats loading
Add progressbar js-based rendering