Add a github_url property on github objects

This property returns the (most relevant) url of the object on github
Add a link to the github issue page on an issue detail
Pull requests comments are now fetchable
Display pull-requests comments
Manage pr comments on outdated diff

pr-comments entry points are now collapsable
- default closed for outdated diff
- else open

+ add the date of the beginning of the discussion
Involved people includes pr-comments authors
Display pr-comments number for each entry-point
Save pr-comments counts and use total_comments_count in templates
Add functionnalities to fetching

- add a way to force github callable identifiers to use
- add a way to allow only create or update
Fetch some infos from pull-requests API
Do not update mandatory FKs if nothing got from API
If PR is merged, display it
Display PR source and enhance header
Update the correct "fetched_at" field for prs
Stop importing a list when min_updated_date is reached
Fetch only 20 items per page when updating a repo

Still 100 when creating
Use different min/max items per page for each model
fetched_at can be compared to other field than updated_at

ie "created_at", even if not used for now
Fetch issues events
Allow passing page and per_page parameters while fetching