feat(sidebar): show welcome view if there are no tokens set

Show a welcome screen in the sidebar the extension doesn't have any access tokens set up.

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The key idea

When there are not tokens, we'll set the extension context to:

vscode.commands.executeCommand('setContext', 'gitlab:noToken', true);
vscode.commands.executeCommand('setContext', 'gitlab:validState', false);

This context is then used by VS Code to show the welcome view. When the extension state is not valid, the following things happen:

1. the issuable data provider (returning results of custom queries) returns []

   if (!extensionState.isValid()) {
     return [];
   }

2. the current_branch data provider gets hidden thanks to this package.json configuration:

   "views": {
     "gitlab-workflow": [
       {
         "id": "currentBranchInfo",
         "name": "For current branch",
         "when": "gitlab:validState"
       }
     ]

   (the current_branch data provider also returns [] for consistency with issuable, but it shouldn't matter)

3. VS Code replaces the content of the sidebar with the welcome view, because the issuable data provider returned [] and because the when condition on the welcome view is met:

   "viewsWelcome": [
     {
       "view": "issuesAndMrs",
       "contents": "Welcome to the GitLab Workflow extension. It seems ...",
       "when": "gitlab:noToken"
     }
   ],

Related to #281 (closed)