Followup: Provide alternative for SQLite

This is basically a revert of some of the changes in "Much faster
approach to retrieve project's resource paths" with some minor changes
because `get_path_sortkey` behaves differently now. Due to the
limitations of SQLite there is no way to do this just using raw SQL.
Cache list of project resources

Fixes evernote/pootle#79.

Pick notes: Fixed minor conflicts and added Signals separator comment.

Conflicts:
	pootle/apps/pootle_project/models.py
Followup: Also cache when using SQLite
Remove margin from root elements
Cleanup: removed dead code

Conflicts:
	pootle/apps/pootle_statistics/models.py
Fixed use of class attribute
Adapt first drop-down elements according to the current context

Fixes evernote/pootle#81.

Pick notes: Just fixed minor conflicts because of code divergence.

Conflicts:
	pootle/core/url_helpers.py
	pootle/static/css/style.css
Redirect to cross-language page for missing resources

Pick note: Just minor conflict because of code divergence.

Conflicts:
	pootle/core/decorators.py
Check for missing resource_path property
Simplified code
Ensure project admin URLs are matched

Conflicts:
	pootle/apps/pootle_project/urls.py
Fixed stupid caching mistake

Conflicts:
	pootle/apps/pootle_project/models.py
Cleanup: make code more readable
More flexible way to customize behavior for each drop-down
No need to determine the initially-selected value

This is already done in `PTL.browser.fixDropdowns` when the `pageshow`
event fires.
Append prefix to context-specific drop-down elements

This workaround is to avoid having `<option>` elements with duplicated
`value` values, otherwise setting its value via select2/jQuery will fail.

Pick notes: Minor conflict because of change done to don't hide the
    resources dropdown in 001c0cba.

Conflicts:
	pootle/templates/core/_breadcrumbs.html
Adjust breadcrumbs visibility after initial elements have been set

Avoids seeing selections being changed after the page has been loaded.
Expression doesn't end here
Use a drop-down menu for navigation instead of tabs

Fixes evernote/pootle#76.

Pick notes: Fixed several conflicts due to cleanups we did, code divergence,
    code we kept, and code we didn't pick. This requires cleanups to
    properly show the dropdown (it is always hidden). Also recovered some
    styling for the breadcrumbs that we didn't pick in the past.

Conflicts:
	pootle/apps/pootle_project/views.py
	pootle/apps/pootle_translationproject/views.py
	pootle/static/css/style.css
	pootle/templates/editor/main.html
	pootle/templates/languages/_tabs.html
	pootle/templates/projects/_tabs.html
	pootle/templates/translation_projects/_tabs.html
	pootle/templates/translation_projects/base.html
Followup: Do not hide the navigation dropdown

Like we did for the resources one in 001c0cba.