Only load full item content when needed

[erikhofer]

Currently, the whole workspace is loaded into memory at once. For some item properties this is needed because they are displayed in many places. The majority of the content, however, is typically only displayed on the item details page.

For large workspaces, it may become necessary to only load the full item content when it is actually needed.

This is especially relevant for the static site export. There, the workspace is not loaded from local disk but transferred over the network.

One problem is populating the full-text search index. Maybe we can at least do this lazily (and cache it somehow?).