2FA checks for Git over HTTP

## What does this MR do?

This MR allows the use of `PersonalAccessTokens` to access Git over HTTP and makes that the only allowed method if the user has 2FA enabled. If a user with 2FA enabled tries to access Git over HTTP using his username and password the request will be denied and the user will be presented with the following message:

```
remote: HTTP Basic: Access denied
remote: You have 2FA enabled, please use a personal access token for Git over HTTP.
remote: You can generate one at http://localhost:3000/profile/personal_access_tokens
fatal: Authentication failed for 'http://localhost:3000/documentcloud/underscore.git/'
```

## What are the relevant issue numbers?

Fixes #13568 

See merge request !5764