Allow user to sync his existing comments with commentor

**Why ?**

Today, Commentor allows you to see the comments that were made after you plugged the webhook on your repo. Users complain they need to see value in using Commentor right after logging in. They would need to be able to get their existing comments, tag them on commentor and see its analysis

**What ?**
This issue focuses on the synchronisation part of existing comments. The idea is to create an API route that will take a project and a period along with the user's oAuth token and perform the required API calls to the target provider in order to get the comment for the project, during a period.

Once this API call is working, it would be great to think of the best moment to trigger it. Either with a button "Sync my comments" [Easy solution - recommended], based on the selected filters.

Keep in mind you have to prevent doublons.

We could think of directly trigering this action whenever a user requests to see comments on a repo with a date filter that does not match any comments on our database.
In this case, we should display an informative loader on the front telling him that we are making a long call to his provider.