Show pending indicator for messages that are being sent (incl retry)

[mahibi]

Messages that are being sent should be visible inside the chat, but there should be an indicator that sending is still in progress.
Or if sending failed, there should be an option to re-trigger sending.

Also see #1259 for an old draft. Now with offline support this may have to be handled differently.

Also make sure to set reference id's for messages that are sent when being offline: #4251.

Basic steps:

(discussed with @Ivansss)

When sending message (also while being offline):

• create temporary message
• set a reference id
• put it into chatlist and show an icon (spinner or clock..)

When receiving send-response or get messages

• check all reference ids
  • if reference id from a received message is equal to a reference id of temporary message
    • replace temporary message with received message
    • delete temorary message from DB

Links:

• https://github.com/nextcloud/talk-ios/blob/master/NextcloudTalk/BaseChatViewController.swift#L480
• https://nextcloud-talk.readthedocs.io/en/latest/chat/#sending-a-new-chat-message