Introduce named routes in brig

[pcapriotti]

This PR is the brig counterpart to #2022, i.e. it replaces uses of servant-generic in brig with Named routes, and splits the API tree into sections.

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.

[supersven]

The translation to the new format looks good. IMHO it's better to read. 👍

Only a few minor questions...

[pcapriotti]

Thanks for reviewing. Unfortunately, since the PR contained a merge of #2091 (to avoid having to deal with conflicts later), you have also looked at some changes from the other PR. I have rebased now.

Fixed the typo and readded the missing Summarys (thanks for spotting that!). Would you mind having another quick look?

[supersven]

Looks great 👍