Change filename to username in toasts about sending thanks (#6467)

This fixes #6466.

Also fix the messages themselves a bit:
* Removed "successfully" from the success message. This word
  is usually redundant, because the message already says that it
  was done. (In MediaWiki, there's a specific convention about it:
  https://www.mediawiki.org/wiki/Help:System_message#Avoid_jargon_and_slanghttps://www.mediawiki.org/wiki/Help:System_message#Avoid_jargon_and_slang
)
* Added a missing preposition to the failure message.

Co-authored-by: Nicolas Raoul <nicolas.raoul@gmail.com>

app/src/main/java/fr/free/nrw/commons/media/MediaDetailFragment.kt

@@ -1027,12 +1027,12 @@ class MediaDetailFragment : CommonsDaggerSupportFragment(), CategoryEditHelper.C
         val message: String = if (result) {
             context.getString(
                 R.string.send_thank_success_message,
-                media!!.displayTitle
+                media!!.user
             )
         } else {
             context.getString(
                 R.string.send_thank_failure_message,
-                media!!.displayTitle
+                media!!.user
             )
         }