[NEW] Initial support to Discussions

[rafaelks]

@RocketChat/ios

This PR adds initial support to Discussions feature.

• User can now see discussions into the list of chats;
• There are specific icons for Discussions;
• Discussions will always use fname property instead of name;
• There's a new chat item to display Discussion information and allow user to open/read it;

[codecov]

Codecov Report

Merging #2646 into develop will decrease coverage by 0.14%.
The diff coverage is 6.82%.

@@             Coverage Diff             @@
##           develop    #2646      +/-   ##
===========================================
- Coverage    26.17%   26.02%   -0.15%     
===========================================
  Files          463      465       +2     
  Lines        17224    17373     +149     
===========================================
+ Hits          4509     4522      +13     
- Misses       12715    12851     +136

Impacted Files	Coverage Δ
Rocket.Chat/Models/Message/Message.swift	88.57% <ø> (ø)	⬆️
...Chat/Controllers/Chat/MessagesViewController.swift	31.49% <ø> (ø)	⬆️
...ontrollers/Chat/ChannelActionsViewController.swift	0% <ø> (ø)	⬆️
...at/Models/Subscription/UnmanagedSubscription.swift	0% <ø> (ø)	⬆️
...ws/Chat/New Chat/Cells/MessageDiscussionCell.swift	0% <0%> (ø)
...Controllers/Chat/ChatSections/MessageSection.swift	4.04% <0%> (-0.12%)	⬇️
...iews/Cells/Subscription/BaseSubscriptionCell.swift	0% <0%> (ø)	⬆️
...New Chat/ChatItems/MessageDiscussionChatItem.swift	0% <0%> (ø)
Rocket.Chat/Managers/AppManager.swift	34.15% <0%> (-5.96%)	⬇️
Rocket.Chat/Models/Subscription/Subscription.swift	48.97% <100%> (+2.17%)	⬆️
... and 7 more

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