Add link to repository overview page on `developer.mend.io`

[HonkingGoose]

Describe the proposed change(s).

Add a link to the repository overview page on the developer.mend.io domain. The link should:

• [ ] Have a good name (no bare link!)
• [ ] Point to the repository's overview page
• [ ] Not be hardcoded into the OSS code, but use a config template

The template link should look like this:

Go to the [Mend Developer Portal for your repository](https://developer.mend.io/github/username/repositoryName).

This helps Renovate users see the status of the Renovate bot.

By default, this link should only appear for Mend-hosted app users on GitHub.

[rarkins]

I think we should try to do this using templating of the Dependency Dashboard issue, so that it can be done by config and not hardcoded into the OSS

[mmlb]

I've been adding comments to the dashboard issue with the link but only as I need it, having it done automatically would be fantastic.