impossible to configure signature

Bug summary

According to the documentation, it should be possible to include a signature.
This can be set up in the account's settings.

The Sign emails checkbox indicates whether a signature will be automatically inserted when a composer is opened. You may enter the signature into the box immediately below. You may use HTML tags to style the text. Switch to a preview of the signature using the buttons to the right. If you leave the signature in the Accounts dialog blank, Geary will use the .signature file in your home directory, if it exists. This file may contain either plain text or HTML markup. In the latter case, the markup will be inserted directly into the composer, without any escaping.

I only have a textbox, no checkbox
(I also don't have a save sent mail checkbox, don't know if that's related.)

Including a .signature file in the home directory did not work either.

About you

Geary version: 1.13
Installation method: PPA >> apt-get
Your desktop: GNOME
Your operating system and version: Ubuntu 18.04
Email provider: Exchange through IMAP

Steps to reproduce

Configure mail account (this works)
Enter account settings

What is the current bug behavior?

no checkbox to enable the signature
text in signature field is not included in new mails
html code in signature field is escaped
~/.signature file not taken into account

What is the expected correct behavior?

include mail signatures in new mails

Relevant logs and/or screenshots