Parse and expose alert's `startsAt`

Parse strictly RFC3339

Present alert's `starts_at` as RFC3339

Expose alert's `starts_at` in the incident summary

Parse `labels/alertname` as alert title

Add missing spec for Alert#description

Add missing specs for Alert#annotations

Add missing spec for Alert#starts_at

Add missing specs for Alert#valid?

Tweak specs for Alert#{gitlab_alert,environment}

Parse full_query from generatorURL

Move creation of incident summary into presenter

Expose alert's full query if available

Adjust specs for alert mails

`startsAt` is now mandatory

Add changelog entry

Remove implicit `nil` in rescue

Define `let` before the `before` block

Inline key-value split

Define value inside the shared example

Improve example naming

Apply review suggestions