Serialisation of remove proposals

[pcapriotti]

This PR adds serialisation of signed remove proposals.

https://wearezeta.atlassian.net/browse/FS-823

Checklist

• [x] The PR Title explains the impact of the change.
• [x] The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• [ ] changelog.d contains the following bits of information (details):
  • [ ] A file with the changelog entry in one or more suitable sub-sections. The sub-sections are marked by directories inside changelog.d.
  • [ ] If new config options introduced: added usage description under docs/reference/config-options.md
  • [ ] If new config options introduced: recommended measures to be taken by on-premise instance operators.