User should be able to start/stop recording specific streams with an Action

From #656:

> I'm thinking of something like having a video recording action that is configurable.
>
> - It could be used without being configured, in which case it would only work if there's only one stream available;
> - If there's more than one stream available and it is not configured, it will trigger start/stop recording on all streams;
> - The user will be able to associate the action to an specific stream, doing it on the video configuration page, or in an actions' configuration page;
> - The user would be able to create many of those actions, one for each stream, to be able to control then individually;
> - Alerts for start/stop of those would be emited so the user can have a feedback about what exactly is going on (e.g.: "Started recording stream 0", "Stopped recording stream 2", "Started recording all streams", etc);

We currently can do the first two (start/stop recording of all streams without configuring the action). We are missing the ability to allow starting/stopping specific streams with configurable actions.

Start by tracing the existing recording action that currently starts and stops all streams, then inspect the video configuration page and actions' configuration page mentioned in the issue. Done means configurable actions can target individual streams or all streams as appropriate, with alerts identifying which stream or streams started or stopped recording.

Written by the indexing model from the issue text.