fix: combine session image and text into single Telegram message

When creating a session with an image, send it as a single SendPhoto
with the schedule text as caption (+ reply markup), instead of two
separate messages. Falls back to two messages if caption exceeds
Telegram's 1024-char limit.

Also adds BatchMessageEditor helper that transparently handles
EditMessageText vs EditMessageCaption depending on whether the batch
message is a text or photo message. Updated all handlers and web
service to use this helper.

Version bump to 1.9.7.

compose.yaml

@@ -17,7 +17,7 @@ services:
       retries: 10
 
   bot:
-    image: git.codeanddice.ru/toutsu/gmrelay-bot:1.9.6
+    image: git.codeanddice.ru/toutsu/gmrelay-bot:1.9.7
     restart: always
     depends_on:
       db:
@@ -30,7 +30,7 @@ services:
       - gmrelay
 
   web:
-    image: git.codeanddice.ru/toutsu/gmrelay-web:1.9.6
+    image: git.codeanddice.ru/toutsu/gmrelay-web:1.9.7
     restart: always
     depends_on:
       db: