Update changelog for 4.18.

debian/changelog

-tails (4.18) UNRELEASED; urgency=medium
-  * Dummy entry for next release.
- -- intrigeri <intrigeri@debian.org>  Tue, 23 Mar 2021 08:55:52 +0000
+tails (4.18) unstable; urgency=medium
+  * Upgrade Thunderbird to 78.9.0-1~deb10u1
+  * Upgrade Tor Browser to 10.0.15 (tails/tails!399)
+    Closes issues:
+      - Upgrade to Tor Browser based on Firefox 78.10 (tails/tails#18252)
+
+    Commits:
+      - Fetch Tor Browser from our own archive.
+      - Upgrade Tor Browser to 10.0.15.
+
+  * Upgrade Intel microcodes to 3.20210216.1~deb10u1
+
+  * Upgrade firmware-linux-nonfree to 20210315-2
+
+  * Remove Poedit (#18236) (tails/tails!395)
+
+    Commits:
+      - Remove Poedit (#18236)
+
+  * Display an error message when the Upgrader fails to download the signing key
+    (tails/tails!390)
+
+    Closes issues:
+      - Display an error message when the Upgrader fails to check for available
+        upgrades (tails/tails#18238)
+
+    Commits:
+      - Display an error message when the Upgrader fails to download the signing key
+
+  * Remove clock sync notification (tails/tails!389)
+
+    Closes issues:
+      - Remove the "clock synchronization" notification (tails/tails#7439)
+
+    Commits:
+      - tails-htp-notify-user only notifies if $problems
+
+  * Bump snapshots of the Debian APT archive to 2021033101 (tails/tails!388)
+
+    Commits:
+      - Bump snapshots of the Debian APT archive to 2021033101
+
+  * APT: use non-onion HTTPS sources for Debian repositories (tails/tails!383)
+
+    Closes issues:
+      - Fix failures with Debian's APT repository onion service (tails/tails#17993)
+
+    Commits:
+      - APT: use non-onion HTTPS sources for Debian repositories
+
+  * Merge base branch earlier (tails/tails!381)
+
+    Closes issues:
+      - The mergebasebranch option merges the base branch too late (tails/tails#12557)
+
+    Commits:
+      - Document that faketime is needed on the host to build Tails.
+      - Vagrant: don't install faketime any more.
+      - Drop useless version part from development build filenames.
+      - Build system: only add base branch info to image names when relevant.
+      - Build system: make development images' names more consistent.
+      - Build system: move around logic to reduce ENV state.
+      - Build system: re-run rake after merging base branch (#12557)
+      - Rakefile: handle jenkins passing the now obsolete mergebasebranch.
+      - Build system: by default, merge the base branch early when building (#12557)
+
+ -- Tails developers <tails@boum.org>  Mon, 19 Apr 2021 06:28:28 +0000
 tails (4.17) unstable; urgency=medium
   * Upgrade Thunderbird to 78.8.0-1~deb10u1