When already a FUNDING.json present, propose editing it instead of creating

[efstajas]

Currently, we always prompt people to create a FUNDING.json file with only information for the current network in it.

We already have a few FIL-RetroPGF-2 recipients that have claimed on Ethereum, and they'll have to claim again on Filecoin. In this case, the app should see that there's already a FUNDING.json file in the repo, and prompt you to add the new lines for the current network, instead of creating an entirely new file. The UI in the code preview could look like a Git Diff in this case, where the existing lines are greyed out, and the newly added ones have little plusses on the left and are green.

The "Add to repo" button should also say "Edit on GitHub" and propose an edit to the existing file instead.

[brandonhaslegs]

Changed the text a bit in certain places where it said "add" to "edit" and also added this green text with green background for the stuff you're supposed to add to the FUNDING.json file.

[efstajas]

Change step description to add "please add an owner address for < Network Name > [...]"