Links aren't parts of drive exports

**Describe the bug**
As of today links created in CryptDrive aren't parts of exports.

**Where did it happen?**
CryptPad.fr

**To Reproduce**
Steps to reproduce the behavior:
1. Go to https://cryptpad.fr/drive/
2. Create a link
3. Download your CryptDrive
4. Find no link in the files

**Expected behavior**
Links should be exported too, maybe under `.txt` format?