Migrate MR and Pipeline status bar items to WebIDE

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This issue si about migrating the MR and pipeline status bar items to common code

Techincal details

The CurrentBranchReferesher is the data source for the "For Current Branch" panel:

It is now dependent on desktop-specific logic. We need to abstract all the dependencies to the GitLabPlatform, namely:

• We need to expose the current branch in the platform
  • In desktop, we read it from Git
  • In WebIDE, it's coming from the config
    • the tricky part is that (AFAIKT) if we open MR in the WebIDE, the ref config is empty (needs verification). That means we would have to get the branch from an MR
    • another tricky part is that the WebIDE currently has a bug, and you can't open tags. The tags are handled differently in desktop Workflow
• All the GitLab service API calls (e.g. getting pipeline and MR) must be changed to use the new fetchFromApi method

Progress:

@ealcantara and I were pairing on abstracting the API calls.