Bumps [composer/installers](https://github.com/composer/installers) from 1.5.0 to 1.6.0.
<details>
<summary>Changelog</summary>
*Sourced from [composer/installers's changelog](https://github.com/composer/installers/blob/master/CHANGELOG.md).*
> # Change Log
>
> ## [Unreleased]
</details>
<details>
<summary>Commits</summary>
- [`cfcca6b`](cfcca6b1b6) Merge pull request [#397](https://github-redirect.dependabot.com/composer/installers/issues/397) from harmenjanssen/normalize-october-plugin
- [`8da283a`](8da283adf7) Normalise vendor directory containing hyphen
- [`5d15e4e`](5d15e4e9aa) Merge pull request [#376](https://github-redirect.dependabot.com/composer/installers/issues/376) from thomscode/disable-installers
- [`78eb8ca`](78eb8ca263) Add use statements in place of fully qualified namespaces
- [`4bff163`](4bff1637ea) Merge branch 'master' into disable-installers
- [`a118c5b`](a118c5bbb5) Merge pull request [#391](https://github-redirect.dependabot.com/composer/installers/issues/391) from davidbarratt/mediawiki-core
- [`2353998`](2353998d5a) Add MediaWiki Core
- [`c3c5297`](c3c5297a3b) Add false as an option to prevent installers from being disabled.
- [`3806135`](38061358f2) Update documentation to include false option
- [`1aa22c1`](1aa22c1756) Merge branch 'master' into disable-installers
- Additional commits viewable in [compare view](https://github.com/composer/installers/compare/v1.5.0...v1.6.0)
</details>
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</details>
We can't display BISAC subjects in a human-readable format, so we should apply the `is_bisac()` utility to ensure that we only attempt to display Thema subjects on the catalog and book cards within the catalog.
This PR lets Aldine (and by extension the Pressbooks landing page site) display the export and erase personal data panels introduced in WordPress 4.9.6. They will only be visible to those who have admin access to the root site (network managers).