Servantify brig properties endpoints

[pcapriotti]

This PR servantifies brig's properties endpoints.

A couple of things worth noting:

• One of the servantified endpoints (namely GET /properties) was used to test /i/metrics. This role has now been taken by /onboarding/v3 (arbitrarily).
• Since property values are validated in a way that requires knowledge of the settings, it cannot simply be done at the level of the Servant type classes. Therefore, I introduced a RawPropertyValue wrapper around a ByteString, which leaves it to the handler to do the actual parsing and validation.

As a side note, it is a bit strange that we are parsing property values at all, since we just convert them back to bytestrings immediately to save them in the database, and never look inside again. In principle, we could make them completely opaque bytestrings and avoid having to deal with all of these complications. However, it is hard to do without changing the API somehow, since these values are used inside other JSON objects.

Checklist

• The PR Title explains the impact of the change.
• 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.

[akshaymankar]

🚀