Fix livechat notifications for guest pool based routing

[mrsimpson]

This fixes #271 desktop notifications not being created if a live chat visitor created a room by sending the first message.

How to test

• Configure a livechat with guest pool, add an agent.
• Create a new live chat and verify the desktop notification appears
• add two departments and configure an agent
• Create a new live chat selecting a department
• Send a new message and check the notification appears

Remarks

• Notifications other than livechat and guest pool are based upon the user's subscriptions. Only for the live chat inquiries (the "requests for an agent to join"), there's a need for an alternative mechanism.
• Once an agent is joined, he's the only one to notify which matches the default: He'll have a valid subscription once joined.
• All code added is non-destructive (new functions, arrays concatenated, ...) - it will most likely not make things worse, even if it doesn't meet all the aspects
• From an architectural perspective, it would be better to have the code for creating the livechat-notifications in a separate hook defined in the livechat-package. The way it's been implemented creates a dependency from the notifications-api to the livechat-models - which is not good. On the other hand, the dependency is de-facto already there (room.t !== 'l') and implementing it there has a high re-use.

[mrsimpson]

PR to RC core addressing RocketChat#7796: RocketChat#10588

[mrsimpson]

Observe RocketChat#10318 which adds even better notification title ;)

[mrsimpson]

@janrudolph can you actually test this?

[janrudolph]

Verified on Mac OS with Chrome (to be repeated on Windows):

• Configure a livechat with guest pool, add an agent.
• Create a new live chat and verify the desktop notification appears

Not tested yet:

• add two departments and configure an agent
• Create a new live chat selecting a department
• Send a new message and check the notification appears

Additionally, I tested the following case:

• Configure Livechat_Routing_Method to Least Amount. Check if notification appears when a new customer writes its first message

[vickyokrm]

Testing comments:

1. Created a guest pool live chat with multiple agents. As per the test steps indicated, there was no notification appeared when the live chat was created. But the notification shows up, when the user write a message.

[vickyokrm]

Scenarios Tested successfully on Mac:

• Configure a livechat with guest pool, add an agent.
• Create a new live chat and verify the desktop notification appears
• add two departments and configure an agent
• Create a new live chat selecting a department
• Send a new message and check the notification appears

[janrudolph]

lgtm
However, there is another issue: #364

[janrudolph]

lgtm