Add "Conversation" and "Communication" metadata to internal parsers

[aberenguel]

Fixes #2686

Parsers to be changed:

• WhatsAppParser
• DiscordParser
• TelegramParser
• ThreemaParser
• SkypeParser

[aberenguel]

@lfcnassif I added new chat metadata ("Conversation:" and "Communication:") to WhatsAppParser, TelegramParser and DiscordParser. In the above commits, some bugs were also fixed. I don't know if it is worth to handle ThreemaParser and SkypeParser right now.

[aberenguel]

Motivation

UFED chats metadata were uses the Conversation: (chats) and Communication: (messages) prefixes, but the internal decoding parsers still used their own parser-specific keys. This PR standardizes those keys so internal and UFED parsers emit the same metadata wherever possible.

Changes

• Shared keys and constants centralized in ExtraProperties and ConversationConstants (conversation id/Name/Type/Account/Participants/Admins, message From/To/Direction/Date, normalized types OneOnOne/Group/Broadcast/Unknown).
• IndexItem now auto-computes the :count attribute for participants/admins.
• WhatsApp, Telegram, Discord, Skype and Threema parsers updated to emit the standardized metadata and conversation types, using the party string builders.
• WhatsApp had to implement fetching of group admins (read from the group tables), which was not decoded before; Telegram already decoded admins and now maps them to the standardized Conversation:Admins key.
• UFED handlers aligned to the same constants and suffix logic.
• WhatsApp and Telegram tests updated for the new keys and party-string format.

The PR also includes other fixes not related to the metadata standardization, bundled in along the way: WhatsApp phone-number resolution, account/admin reading and a quoted-message CSS fix; Telegram chat-name handling and user-account retrieval; and ignoring invalid media hashes. Conversation type mapping is per-app (e.g. WhatsApp channels map to Broadcast).

This is a broad change touching many parsers and their tests, so it is good to review.