let a fork to be relanded

[alexcohn]

This is mainly a UI feature request, related to #1094.

Anyways, currently my fork can compare, issue Pull Requests, etc. with any other fork of the same fork tree. But there is one that is more equal than others: the fork that I originally used to create my fork.

It happens once in a while, that one or another related fork becomes more important in my work. It would be nice to have an option to mark this change in repository Settings, so that it is reflected on the repository main page as forked from, and become the first suggestion for Pull Requests and compare.

If and when #121 and/or #1122 get implemented, this will become more than a UI feature.

[TPS]

@alexcohn #121 is now implemented, so now you might interest @botatoes, &c, in this.