Add ability to register runners under repository for individual accounts

[mattia]

This PR adds the ability to register runners for a repository of an individual account.
To be able to distinguish between runners for an organization and for repository of an individual account a new GitHubRunnerScope is added.

The "GitHub" tab in the settings is updated so both scope types can be configured, but only one scope can be active at the time (the style of the form page is a little bit changed, to try grouping together informations)

This changes are backward compatible as the default runner scope is organization; unfortunately I'm not able to test this changes on a real organization so a real test should be performed.

Fixes #5

[mattia]

(If this PR gets merged, maybe the "Configuring Tartelet" section of the wiki should be updated but I don't think it can be done via a PR)

[simonbs]

@mattia Thanks for making these changes and opening a PR!

I'm in the process of reviewing the changes and just wanted to make a note for myself and anyone else seeing this that it seems the permissions needed in the GitHub App are:

• Repository Permissions: Administration (Read and Write)
• Repository Permissions: Metadata (Read-only)

[simonbs]

I have verified that the changes work for repositories owned by an individual and that the existing functionality still works for organization accounts 🎉

[simonbs]

Looks good! I'll make a PR shortly after this that streamlines the appearance of settings to match the grouped form style introduced in this PR. I didn't know about that style but we should totally adopt it 😃