Add review-apps support

[bohdan-shulha]

This feature highly depends on #255 .

We need to add a new tab to the "services page" - "Environments" (or "review apps", as it is going to be not strictly an "environment").

Clients should be able to connect their GitHub / GitLab / Whathevergit repository management tool and a new review app should be created each time a PR is made.

Review apps should be destroyed once the PR is removed.

Users should be able to specify for how long to keep the review apps up and running for PRs without updates for some period (like, removing apps after 1 week of inactivity).

Probably, we'll need to include the auto start/auto sleep functionality into the scope of this ticket, depending on the amount of work that has to be done.

Technical

To simplify infrastructure, APIs, database architecture, UX all review apps will be created per-worker. This way, users will be able to more easily start specific workers depending on, for example, files being changed.