Granular global search/diff matching arbitrary policy block subtrees

### Problem description
#2631 introduced capability to globally search for 'similar' policies produced by different Guardian instances and/or different Standard Registries (SRs). For policy creators it may be useful to make such search/diff more granular, i.e. to have facilities to search based on an arbitrary selection of blocks inside a polcy. I.e. to find 'matching' policies containing similar set of blocks, even if the rest of the policy differs.

### Requirements
Introduce the facility in the Policy Editor view to select a group of blocks and globally search for policies containing similar set of blocks. See the screenshots below:



### Definition of done

- Policy creation and publishing also publishes block 'index' for the policy which enable search as described above
- This happens transparently to users/authors who are not required to do any additional actions
- The existing UI dialogues and pages are updated to enable the global and local search scopes

### Acceptance criteria
The search returns results within maximum 10-15 seconds.