Hide fediverse creator when unused, and allow a globally-set value. (#931)

### Prerequisites

Put an `x` into the box(es) that apply:

- [ ] This pull request fixes a bug.
- [x] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

The new-ish changes to support fediverse creator tags seemed to
introduce two undesirable (to me) features.

1. People who didn't use the feature got an empty meta tag added to
every page with no value.
2. People who own their site and are the only author of content to it
needed to add the value on every single page to get it to function.

This change allows for a default value to be set, and removes the tag if
it is valueless.

### Issues Resolved

#930 

### Checklist

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#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [x] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [ ] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [ ] Add yourself to `CONTRIBUTORS.md` if you aren't on it already