services/horizon: Sort trustlines by asset code and issuer in GET /acounts/{id}

[2opremio]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Followup from #2516 , Fixes #2447

#2516 sorts the balances for GET /accounts but not GET /accounts/{id}. This PR ties the loop.

Why

horizon-cmp outputs a large amount of false positives because of this, plus it's always a good idea to generate a stable output, independent of the Horizon server used.

Known limitations

The DB query becomes slower. I pondered ordering it on the Horizon-side, but I figured a DB engine would be more efficient at this.