fix: Improve CSV badge awarding with validation and error reporting

[Marvinrose]

Fix: CSV badge awarding with validation and error reporting

Fixes #348

Problem

The existing award_from_csv() had multiple issues:

• In Python 3, file lines are bytes not str, causing .split(",") to silently fail
• No validation for missing or empty file uploads
• No feedback to the admin on what happened, the original requester specifically asked to know which accounts weren't found
• No handling of malformed CSV lines

Changes

• Added .decode("utf-8") fix for Python 3 bytes handling
• Added file presence and empty filename validation
• Added detailed summary flash messages covering:
  • Successful awards count
  • Users that didn't exist and were auto-created
  • Skipped duplicates (already awarded)
  • Badge IDs not found in the system
  • Malformed lines that couldn't be parsed
• Added tests

[Marvinrose]

Hi @gridhead,
I noticed after submitting this, that award_from_csv in tahrir/views/admin.py is a part of the legacy code scheduled for removal in #904. Would you prefer I close this and instead implement CSV badge awarding as a new endpoint in tahrir/endpoints/?