Add MFSA id.

debian/changelog

 tails (3.8) unstable; urgency=medium
 
   * Security fixes
-    - Upgrade Tor Browser to 7.5.6 (Closes: #15683).
+    - Upgrade Tor Browser to 7.5.6 (MFSA 2018-17; Closes: #15683).
     - Upgrade Enigmail to 2.0.7 (partly fixes #15602 aka. EFAIL).
     - Upgrade libgcrypt to 1.7.6-2+deb9u3 (DSA-4231-1).
     - Upgrade perl to 5.24.1-3+deb9u4 (DSA-4226-1).

wiki/src/security/Numerous_security_holes_in_3.7.1.mdwn

@@ -8,7 +8,7 @@ Several security holes that affect Tails 3.7.1 are now fixed in Tails 3.8.
 We **strongly** encourage you to
 [[upgrade to Tails 3.8|news/version_3.8]] as soon as possible.
 
- - Tor Browser: [[!mfsa2018 XXX]]
+ - Tor Browser: [[!mfsa2018 17]]
  - Enigmail: [[!cve CVE-2018-12020]], [[!cve CVE-2018-12019]]
  - libgcrypt [[!debsa2018 4231]]
  - perl [[!debsa2018 4226]]