Remove StockMessage.action_by_contact() (#3518)

Use separate strings for "... by me" and "... by contact"
to make them easier to translate.

CHANGELOG.md

@@ -42,6 +42,7 @@
 - jsonrpc: add `BasicChat` object as a leaner alternative to `FullChat` #3590
 - jsonrpc: add `last_seen` property to `Contact` #3590
 - breaking! jsonrpc: replace `Message.quoted_text` and `Message.quoted_message_id` with `Message.quote` #3590
+- add separate stock strings for actions done by contacts to make them easier to translate #3518
 
 ### Changes
 - order contact lists by "last seen";