Adds `certs-set` and `certs-remove` plugin triggers so other plugins can install or remove an app's SSL cert/key pair without shelling out to the `dokku certs:add` / `dokku certs:remove` subcommands. Shared implementations live as `fn-certs-set` and `fn-certs-remove` in `plugins/certs/internal-functions`, with the subcommands and the new triggers calling `verify_app_name` before delegating.
This standardization makes it somewhat easier to read stacktraces as the command names are all uniform, so it will be slightly easier to scan trace output.
While I do not agree with _every_ style change, this will force Dokku to have consistent formatting across all shell scripts, which is arguably a Good Thing™.
The command used to reprocess everything is:
```shell
shfmt -l -bn -ci -i 2 -w .
```
