ContentBundle cleanups

Share get_installation_time() between all bundle types so that it works
with content bundles.

Add get_tags() stub, this is called by Sugar.

Remove unused fields: l10n, category, subcategory, etc.

We remove support for running OLPC's content bundle indexer.
This is assumed to go away now that Sugar will support content bundles
as first-class objects.

Some minor behavioural fixes and cleanups.

This now implements http://wiki.sugarlabs.org/go/Content_bundles
and when care is taken to include all the fields required by old
Sugar versions, there is no change in compatibility of existing or
new content bundles.

Required for http://wiki.sugarlabs.org/go/Features/Content_support
Icon: handle image loading errors

If the icon can't be found, fall back on showing a generic icon.

Useful when activity/content bundles specify a bad icon, which is
unfortunately true for most content bundles in current existance
(because the icon field was previously unused).