wordpress: remove bundled plugins and themes

Wordpress bundles some non-essential plugins and themes, then pesters
users to upgrade them. As we make the whole webroot readonly, it is
not possible to trivially delete them. Instead we should have users
explicitly install plugins via the existing nixos module.

nixos/doc/manual/from_md/release-notes/rl-2305.section.xml

···
       </listitem>
       <listitem>
         <para>
+          The <literal>wordpress</literal> derivation no longer contains
+          any builtin plugins or themes; these can be found in
+          <literal>wordpressPackages.{plugins,themes}</literal>,
+          respectively.
+        </para>
+      </listitem>
+      <listitem>
+        <para>
           <literal>llvmPackages_rocm.llvm</literal> will not contain
           <literal>clang</literal> or <literal>compiler-rt</literal>.
           <literal>llvmPackages_rocm.clang</literal> will not contain

nixos/doc/manual/release-notes/rl-2305.section.md

···
 
 - `tut` has been updated from 1.0.34 to 2.0.0, and now uses the TOML format for the configuration file instead of INI. Additional information can be found [here](https://github.com/RasmusLindroth/tut/releases/tag/2.0.0).
 
+- The `wordpress` derivation no longer contains any builtin plugins or themes; these can be found in `wordpressPackages.{plugins,themes}`, respectively.
+
 - `llvmPackages_rocm.llvm` will not contain `clang` or `compiler-rt`. `llvmPackages_rocm.clang` will not contain `llvm`. `llvmPackages_rocm.clangNoCompilerRt` has been removed in favor of using `llvmPackages_rocm.clang-unwrapped`.
 
 - The EC2 image module previously detected and automatically mounted ext3-formatted instance store devices and partitions in stage-1 (initramfs), storing `/tmp` on the first discovered device. This behaviour, which only catered to very specific use cases and could not be disabled, has been removed. Users relying on this should provide their own implementation, and probably use ext4 and perform the mount in stage-2.

pkgs/servers/web-apps/wordpress/generic.nix

···
   installPhase = ''
     runHook preInstall
 
+    # remove non-essential plugins and themes
+    rm -r wp-content/{plugins,themes}
+    mkdir wp-content/plugins
+    cat << EOF > wp-content/plugins/index.php
+    <?php
+    // Silence is golden.
+    EOF
+    cp -a wp-content/{plugins,themes}
+
     mkdir -p $out/share/wordpress
     cp -r . $out/share/wordpress