Store validator and nominator slashes in db

[mariopino]

Closes #132

Description:

Store validator and nominator slashes in db for new eras.

Add tables:

• validator_slashes_era
• nominator_slashes_era

NOTE: We need to implement fetch and store of rewards and slashes for previous eras (up to history depth) but not in this PR.

Thank you! 🚀

---

For contributor:

• [ ] Reviewed Files changed in the github PR explorer
• [ ] Attach screenshots of the UI components on the PR description (if applicable)
• [ ] Scope of work approved for big PRs

For reviewer:

• [ ] Manually tested the changes on the UI