Add a "favourites" tasks list

[JeanMertz]

Add the possibility to "favourite" one or more pipelines, for more convenient access.

There are two ways to go with this:

1. We could simply make favoured pipelines be distinct in their representation, and always show them as the top search results

2. Or, introduce a second column that is shown next to the default results, so that more pipelines are visible on screen.

For now, I think the first solution is the simplest to implement.

Similar to #8, the biggest hurdle right now is that the server is session-agnostic, so it cannot know which pipelines you've marked as favourites.

I think for now, storing this locally is fine, it just means that we'll have to sort the list of pipelines client-side, but we're going to have to do that for #8 anyway, so that's not a big deal.