Issue Summary

When an agent user is added to the Slack agent channel, after that user is removed from the agent channel with following message immediately.

@<user name> 
isn't an agent in your <project name> service project, so we've removed them from this channel and sent an access request to your admins.

This is happened if the site has many projects and the user has access for more than 1000 projects.

Currently, there is 1000 projects limits which backend system can retrieve the user's projects.
So if the user has access for more than 1000 projects as an agent (permission: SERVICEDESK_AGENT, BROWSE_PROJECTS, ASSIGNABLE_USER), system fails to get the user's permission for the project. Then Assist remove the user from the channel.

Steps to Reproduce

1. The site has more than 1000 projects and user has access for more than 1000 projects.
2. Add that user to the agent channel
3. Assist app remove the user from agent channel

Expected Results

Agent user is added to the agent channel successfully.

Actual Results

Agent user is removed from the agent channel as soon as the user is added

Workaround

Decrease the number of projects that the user has access. Once the user can be added to a channel, the user's permissions can be reverted.