Fix broken links in some Instance-level integrations

What does this MR do?

Issue #210528 (closed)

We are going to allow users to setup integrations (previously called project services) on the instance (admin) level. This is going to live alongside Service Templates and once all is stable, we will deprecate Service Templates.

This MR fixes a few of the integrations that give errors on the instance-level due to some code being dependent on @project.

• Copy changes in the help text for Mattermost slash commands / Slack slash commands integrations.
• Fix form paths to use re-usable methods across project / admin.
• Fix Alerts integration to be display with placeholders.
• Hide integration button on Slack application integration.

Screenshots

Integration	Project	Admin
Alerts
Mattermost slash commands
Slack slash commands
Slack application

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team