Implement ui to support multiple remotes

[BoscoCHW]

@fcollonval

I implemented a dialogue to manage remote repositories.

[github-actions]

👈 Launch a binder notebook on branch BoscoCHW/jupyterlab-git/issue-1142

[fcollonval]

Thanks for this one. This looks pretty good.

I added some suggestions. As discussed at the meeting, I let you propose something to display the dialog only if the branch has no remote or when triggered from the menu.

[fcollonval]

Thanks @BoscoCHW

Nice work of getting a pure React and a widget dialogs within this PR.

On the code side, would you mind adding tests (in particular Python tests for the new handlers).

On the design side,

What about renaming the menu entry Manage Remote Repositories instead of Add Remote Repository?
About the remote dialog:

What about moving the Add button below the input fields and remove the footer (with the two buttons)?
As it takes some time to retrieve the remote list, what about differentiating state.existingRemotes === null and state.existingRemotes.length === 0. So you can inform the user the remotes list is being retrieve when the state is null. And that there is no remote if the list is empty?

For the advanced push dialog, could you also inform the user that the remote list is being retrieved to reduce the popping of the list?

[fcollonval]

Thanks @BoscoCHW

I propose to do a bit more refactoring of the command. But every is there.

We can discuss this PR more tomorrow.

[fcollonval]

Let's refactor this a bit. Would you mind renaming the gitAddRemote command to be gitManageRemote.

We then should remove the ability to pass args to the execute method. Then in the dialog we should do the addition without closing the dialog. So the dialog is respecting its contract of management not of single action.

[BoscoCHW]

Renamed the command to gitManageRemote and disabled arguments for the command.
To-do:
Implement adding a new remote without closing the dialog

[fcollonval]

Thanks for clean-up the tests and adding new one @BoscoCHW

I think is time to get this in 😉