feature/improve GitHub App installation

[kikkomep]

This PR introduces a new callback endpoint for the LifeMonitor GitHub App.

The endpoint provides:

• a feedback to confirm that the installation was successful;
• an overview of the user's repos where the GitHub App has been installed (with a link to easily jump to those repos on GitHub);
• three additional control buttons to jump to the LifeMonitor Dashboard, to the GitHub App Global Settings and to the App administration page on GitHub (to add/remove repos to the current installation or to completely uninstall the app from the user account.

"Settings" Button

"Manage" Button

By clicking the Manage button the user will be redirected to the GitHub app administration page:

(fix #253, #254)

[kikkomep]

Looks great! The only change I'd make is to have all the links in the feedback page (including the folder icons that link to the repositories) open in new tabs.

fixed in https://github.com/crs4/life_monitor/pull/270/commits/c068f89ae539000cf9167aef2bdbfe01aef87157