Implement Slack command to trigger Analytics integration for a specific site (SITES-20863)

**Is your feature request related to a problem? Please describe.**
Customer wants to integrate Analytics (AA, CJA) for a specific site.

**Describe the solution you'd like**
Add Slack command:
```
Integrate Analytics
Usage: integrate analytics {site}
Integrate analytics for a previously added site
```
That will return a link to the UI where the customer can consent to the proposed analytics implementation and trigger it.
The link needs to have the following query params set with values specific for the site:
- siteId
- gitHubURL
- baseURL
The link to the UI needs to be configurable globally.

**Describe alternatives you've considered**
Manually offering the parametrized link to the customer

No file or test is named. Start by locating the Slack command handlers and the existing site and Analytics integration flow, then trace how UI links are configured. Done means `integrate analytics {site}` accepts a previously added site and returns a configurable UI link containing siteId, gitHubURL, and baseURL.

Written by the indexing model from the issue text.