Integrated Alerting: Validate Slack channel names in Notification Channels

Impact on the user:

• The user is confused and might not get a notification in slack

Steps to reproduce:

1. Enable IA
2. Go to Notification Channels
3. Add slack channel with "#" in the channel name 

Actual result:

• added to the system

Expected Result:

• a warning about the format. something like : "no # needed "
• validation against the name

Workaround:
N/A

Original report:

It is possible to create a notification channel for Slack with channel name #foo. We either should validate channel names and reject ones starting with #, or clean them in API handler.