Allow maintainers to opt-out of "top-level" mentions

From https://github.com/commercialhaskell/stackage/issues/6237 but also requested before. Filing it here for visibility, but the actual change should be made in https://github.com/commercialhaskell/curator

Currently our templated output from curator looks like this

```
lucid-2.10.0 ([changelog](http://hackage.haskell.org/package/lucid-2.10.0/changelog)) (Chris Done @chrisdone) is out of bounds for:
- [ ] servant-lucid-0.9.0.2 (>=2.9.8 && < 2.10). Haskell Servant . @haskell-servant. Used by: library
```

We ping the maintainer of lucid to notify that the package is being held back, but some maintainers want to opt out of pings where they don't need to take action (the action should be taken by the maintainers that depend on lucid). I recall that the opposite is also true, some maintainers want the ping just to be made aware.

I propose that we add a new section to build-constraints.yaml

```
remove-top-level-pings: # name pending
- @githubUser
- @bergmark
```

which would essentially just filter out the github user name from the first line, turning it into

```
lucid-2.10.0 ([changelog](http://hackage.haskell.org/package/lucid-2.10.0/changelog)) (Chris Done ) is out of bounds for:
- [ ] servant-lucid-0.9.0.2 (>=2.9.8 && < 2.10). Haskell Servant . @haskell-servant. Used by: library
```