[NEW] Allows agents to send chat transcript to omnichannel end-users

[renatobecker]

This PR adds to agents the ability to send the chat transcript to end-users, even during the conversation. Currently, only Livechat users can request the chat transcript when the conversation ends, but the Omnichannel solution supports other entry points where there is no interface for end-users to request the chat transcript, so Agents need to send it on their side as well.

In summary, this implementation:

• Adds new permission to allow/deny agents to send the chat transcript;
• If the agent who is assisting the chat is allowed to use the new feature, he/she will be able to send the chat transcript of all closed conversations to the end-user's email who is part of the chat;
• Also, it'll be possible to request the chat transcript even during the conversation. In this case, the chat transcript will be sent as soon as the conversation ends;
• Store a system message for each transcript request sent;

New button to request the chat transcript:

If the conversation is closed, the agent will be able to send the chat transcript immediately...

...and a system message will be displayed in the room:

If the conversation is not closed yet, the agent can also request the chat transcript:

The transcript request data will be displayed in the Visitor Info Panel:

If the chat transcript is no longer needed, the agent can cancel its sending during the conversation:

[renatobecker]

@rodrigok just pushed the fixes.