Add option `filePermissionBits` to override permissions on locally-stored files

What does this PR do?

Adds a new configuration option filePermissionBits to control the file permission bits on uploaded files. This is useful for custom deployments using local file storage, and simplifies use of external tools and media servers.

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.config/ci.yml

@@ -229,3 +229,8 @@ checkActivityPubGetSignature: false
 
 # Upload or download file size limits (bytes)
 #maxFileSize: 262144000
+
+# CHMod-style permission bits to apply to uploaded files.
+# Permission bits are an integer representing User/Group/Other permissions.
+# This setting is only useful for custom deployments, such as using a reverse proxy to serve media.
+#filePermissionBits: '644'