Tags: improve the tags page
Improve the sub-tags listing
Tag: Remove parent relationship

This is not useful/neede for the moment. This can be useful later on to present
a tree of tags.
Tag: move browse function into the tag namespace
Add example yaml configuration
Tag: Show a random thumbnail when browsing
Import: cache the tags

With the SQlite database this does not change the speed of import. This should
improve import speed for PostgreSQL or MySQL.