Shorten prefix to 2 characters

Using 2 hex characters [0-9a-f] will give us 16 * 16 = 256 root folders which
seems more manageable than 4096 (16^3). Assuming a user has 10,000 attachments,
they should roughly distribute at ~40 per folder with prefix length 2 rather
than ~2.5 per folder with a prefix of 3.

app/types/attachment/write_attachment_data.js

@@ -35,6 +35,6 @@ exports._getAttachmentName = () => {
 //      _getAttachmentPath :: Unit -> IO Path
 exports._getAttachmentPath = () => {
   const name = exports._getAttachmentName();
-  const prefix = name.slice(0, 3);
+  const prefix = name.slice(0, 2);
   return Path.join(prefix, name);
 };

test/app/types/attachment/write_attachment_data_test.js

@@ -12,7 +12,7 @@ const {
 } = require('../../../../app/types/attachment/write_attachment_data');
 
 
-const PREFIX_LENGTH = 3;
+const PREFIX_LENGTH = 2;
 const NUM_SEPARATORS = 1;
 const NAME_LENGTH = 64;
 const PATH_LENGTH = PREFIX_LENGTH + NUM_SEPARATORS + NAME_LENGTH;