Dovecot: Add IMAP capabilities.

Nextcloud Mail is having issues with the following Dovecot error:
```
failed: Invalid quoted-printable input trailer: '=' not followed by two
hex digits
```
From
https://forums.cpanel.net/threads/dovecot-invalid-quoted-printable-input-trailer.648081/
it appears that some capabilities are missing. Add them.

dovecot/patch.diff

@@ -196,6 +196,15 @@
  # namespace settings, you can do it by defining namespace sections.
 --- conf.d/20-imap.conf	2020-12-22 13:26:52.000000000 +0000
 +++ conf.d/20-imap.conf	2022-12-05 07:33:47.876334736 +0000
+@@ -35,7 +35,7 @@
+ 
+ # Override the IMAP CAPABILITY response. If the value begins with '+',
+ # add the given capabilities on top of the defaults (e.g. +XFOO XBAR).
+-#imap_capability = 
++imap_capability = +I18NLEVEL=1 SORT=DISPLAY THREAD=ORDEREDSUBJECT THREAD=REFERENCES CONTEXT=SEARCH
+ 
+ # How long to wait between "OK Still here" notifications when client is
+ # IDLEing.
 @@ -95,5 +95,5 @@
  
    # Maximum number of IMAP connections allowed for a user from each IP address.