Ease updating component index of existing sources

Motivation

When you have an existing source and you add new components, you want an easy way to update.

Proposed resolution

Add list of sources on top of the list of components. Make a small table, listing sources and for each source an operation button "Update components", a protected link and route, which triggers the registration of the component index.

We can determine the list of sources by going through all components, as we already do for the unregister-link by source. Let's move this up as another operation button and remove the sources listing in the bottom.