Add IMAP capabilities instead of overwriting them

I wanted to add `COMPRESS=DEFLATE`,
but it should be added only for sessions
that are logged in because `COMPRESS`
command does not work before logging in.

Dovecot already does it correctly
if we don't overwrite the capability string.

CHANGELOG.md

@@ -23,6 +23,9 @@
 
 - set CAA record flags to 0
 
+- add IMAP capabilities instead of overwriting them
+  ([#413](https://github.com/deltachat/chatmail/pull/413))
+
 
 ## 1.4.1 2024-07-31