API: Use default Language manager for LanguageResource

This way it is possible to access to all languages and not just those that
are used on at least a translation project.
API: Complete ProjectResource and add docs

Fix bug 2951
API: Complete LanguageResource and add docs

Fix bug 2952
API: Complete TranslationProjectResource and add docs

Fix bug 2953
API: Add StoreResource and docs

Fix bug 2954
API: Add UnitResource and docs

Fix bug 2955
API: Add SuggestionResource and docs

Fix bug 2956
API: Provide statistics through /resource/pk/statistics/ like URLs

The following resources are now subclasses of StatisticsModelResource:

* LanguageResource
* ProjectResource
* StoreResource
* TranslationProjectResource

Also added new docs for this feature on all the affected resources

Added necessary methods on Project, Language and TranslationProject models

Fix bug 2957
Replace Fancybox with Magnific-Popup

This only switches the plugin, doesn't make any other code changes.
Removed unused template snippets

The unused functionality was removed some time ago in 371d7bcf6.
Replaced Fancybox functionality with Magnific-Popup

Fixes bug 2929.
Added explicit logout link in the agreement box

Fixes bug 2932.
Update UI elements when going back in the browser history

Since browsers implement a back/forward cache, when a page is loaded
from the cache breadcrumbs won't reflect the page's current status.
Therefore it's necessary to alter its state when the page is loaded from
the cache. Unfortunately all browser's don't properly support `pageshow`
event's `persisted` property, so we are running this code for every page
load.

Fixes bug 2949.
Adjusted shadow for forms displayed in lightbox popups
Fixed select2 value setting call
Clarify database migration mention in upgrade docs

Goes some way to addressing the issues mentioned in
http://markmail.org/message/im6xt2fw5xewgffc
Typo and fix exit status
Added format.
Generally disable spellcheck quality check

Fixes bug 2997.
Typos and cleanup: apply styleguide rules for docstrings