services/horizon/internal/actions: Refactor accounts to use new actions system.

[abuiles]

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.

Summary

Use the new action handling pattern on /accounts. This removes the tight coupling between the signers query parameter and the action. It will allow us to implement new functionalities like #442.

Goal and scope

• Remove old action handling pattern for /accounts.
• Use the new action handling pattern as described in Decide on Horizon's actions package #1606
• Generalize the action handler for /account to make it easy to add extra filters.

Summary of changes

• Remove code for old action handler pattern.
• Introduce the new action handler pattern for /accounts.

Known limitations & issues

• N/A

What shouldn't be reviewed

• N/A

[tamirms]

@abuiles I think you need to set the PR to merge into the release branch instead of master.

[abuiles]

@tamirms updated -- fixing the conflict.

[tamirms]

looks good to me!