Add a task to check for updates periodically

It must be enabled and configured by the admin.

Refs: #990

modules/joex/src/main/resources/reference.conf

@@ -180,6 +180,62 @@ docspell.joex {
     }
   }
 
+  # A periodic task to check for new releases of docspell. It can
+  # inform about a new release via e-mail. You need to specify an
+  # account that has SMTP settings to use for sending.
+  update-check {
+    # Whether to enable this task
+    enabled = false
+
+    # Sends the mail without checking the latest release. Can be used
+    # if you want to see if mail sending works, but don't want to wait
+    # until a new release is published.
+    test-run = false
+
+    # When the update check should execute. Default is to run every
+    # week.
+    schedule = "Sun *-*-* 00:00:00"
+
+    # An account id in form of `collective/user` (or just `user` if
+    # collective and user name are the same). This user account must
+    # have at least one valid SMTP settings which are used to send the
+    # mail.
+    sender-account = ""
+
+    # The SMTP connection id that should be used for sending the mail.
+    smtp-id = ""
+
+    # A list of recipient e-mail addresses.
+    # Example: `[ "john.doe@gmail.com" ]`
+    recipients = []
+
+    # The subject of the mail. It supports the same variables as the
+    # body.
+    subject = "Docspell {{ latestVersion }} is available"
+
+    # The body of the mail. Subject and body can contain these
+    # variables which are replaced:
+    #
+    # - `latestVersion` the latest available version of Docspell
+    # - `currentVersion` the currently running (old) version of Docspell
+    # - `releasedAt` a date when the release was published
+    #
+    # The body is processed as markdown after the variables have been
+    # replaced.
+    body = """
+Hello,
+
+You are currently running Docspell {{ currentVersion }}. Version *{{ latestVersion }}*
+is now available, which was released on {{ releasedAt }}. Check the release page at:
+
+<https://github.com/eikek/docspell/releases/latest>
+
+Have a nice day!
+
+Docpell Update Check
+"""
+  }
+
   # Configuration of text extraction
   extraction {
     # For PDF files it is first tried to read the text parts of the