admin: Improve loading speed of details page

The improvements are the following:

1. Do not check if <Item>Association has a qs attribute by evaluating
it, since this will actually evaluate querysets, and for potentially
large querysets we do not want this.
2. Use `prefetch_related` to get the resources along with the project
policies, thereby reducing significantly the number of queries.
3. Use a resource cache to cache the results of Resource queries, since
they amount to almost half of the queries.

With the above improvements, the queries where reduced by 40%.
Merge branch 'feature-admin-improve-speed' into feature-admin