Implement Proper Block Validation for Company Blocks

[zupzup]

Now that the company chain is more complex, implement a similar scheme for Block validation as for Bills

• validation.rs in company chain, which validates new blocks based on the chain state (e.g. if someone is already invited, can't invite again, can't remove someone who is not in the company etc. etc.)
• Use validation for incoming blocks and for API calls
• Implement signer validation
  • Check that the invitee of e.g. InviteSignatory is the signer of the block etc. (same as bills - verify_signer)
  • Use for incoming blocks
• Add validation, that signer of new blocks has to be an accepted & proven signer of the company
• Also add those checks for identity chain