Update CustomAlerts to include Matomo domain with idSite, #PG-4676

[AltamashShaikh]

Description

Update CustomAlerts to include Matomo domain with idSite

Issue No

#PG-4676

Steps to Replicate the Issue

1. Create a new alert and you should see Slack enabled in "Send alerts via" section.
2. Create a new alert, which can be triggered like unique_visitors < 2
3. Execute this command to run alerts ./console core:run-scheduled-tasks --force "Piwik\Plugins\CustomAlerts\Tasks.runAlertsDaily_{alertID}
4. You will see the alert has no link to Matomo and idSite
5. Checkout this PR and redo step 1-4 and you will see an alert with link to Matomo and idsite.

Checklist

• [✔] Tested locally or on demo2/demo3?
• [✔] New test case added/updated?
• [NA] Are all newly added texts included via translation?
• [NA] Are text sanitized properly? (Eg use of v-text v/s v-html for vue)
• [✔] Version bumped?

[james-hill-matomo]

I spent way too long thinking about this bit. I think the upstream code could be improved!

I'm not 100% sure getPiwikUrl will always include the trailing / either, but I couldn't figure it out for sure!

[AltamashShaikh]

@james-hill-matomo We have this code for GA and SEKP plugin and its running fine without any errors so I didn't bother adding / check.

[james-hill-matomo]

We really need a central location for this stuff!