Indexing lifecycles hooks

**Is your feature request related to a problem? Please describe.**
Some functionality would require code to be run before/after ES indexing.

**Describe the solution you'd like**
Indexing should supply hooks that can be used to execute. Proposed hooks:
- post-index
- pre-index (optional/debatable)

Possible solutions: Django signals. If we want to fire these from models, requires #1533

**Describe alternatives you've considered**
Just adding the code on index requests.

**Additional context**
Would be used in e.g. #1535

Start by tracing the existing Elasticsearch index requests and review the related issues #1533 and #1535. Define and implement the proposed post-index hook, resolve whether a pre-index hook is needed, and verify that indexing callers can execute the hooks as intended.

Written by the indexing model from the issue text.