Add display-name option to customize/disambiguate output

[awf]

This PR introduces an optional display-name field in the repo configuration, which allows the user to override the default parent/repo format. This is useful, for example, when indexing the same repo with different config options (e.g. main branch and feature branch).

E.g. this config file:

{
  "dbpath" : "db",
  "vcs-config" : {
    "git": {
      "ref" : "main"
    }
  },
  "repos" : {
    "Hound" : {
      "url" : "https://github.com/awf/hound.git"
    },
    "Hound/alpine" : {
      "url" : "https://github.com/hound-search/hound.git",
      "display-name" : "Hound (alpine)",
      "vcs-config" : {
        "git": {
          "ref" : "alpine"
        }
      }
    }
  }
}

produces this output (the red text) from the CLI client

and this on HTML

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

The PR fulfills these requirements:

• All tests are passing?
• New/updated tests are included?
• If any static assets have been updated, has ui/bindata.go been regenerated?
• Are there doc blocks for functions that I updated/created?

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[salemhilal]

This seems easy and reasonable! Let me get approval from some of the other maintainers before merging it. Thank you for contributing!

[jvt]

This looks great, awesome quality-of-life improvement! Thank you for contributing!