Support entering channels in new integrations form "trigger" section, and apply to GitLab for Slack app integration

About

We have a new integration form that displays the integration form in sections.

GitLab for Slack, Mattermost and Slack Notifications integrations allow the user to select a trigger and also enter channel names of where to post notifications to.

Problem

The Trigger section of the form currently does not support entering channel names next to the triggers.

We would like to use the new integrations form view for these integrations as the UI looks better.

Solution

The new integrations form should support users entering channels in the "trigger" section, when an integration has channel name fields, in a similar way to how the current (old) form does.

The channel name input field should be disabled unless the trigger checkbox has been checked.

We should roll this out for the GitLab for Slack app integration first. An example merge request that enabled the new integration form on the backend !104919 (closed), once the new integrations form can support channel names in the triggers section the change to roll it out to GitLab for Slack will resemble that merge request, so use it as a guide.