Allow for customizable GitHub status context

:wave: Howdy!

We're using bors-ng for a private repository (so I'm not as concerned with consistency across open source projects, since I know that's likely a support nightmare) and thought it'd be nice to be able to control the "context" string used when pushing status checks. This feature is similar to the configurable command trigger.

For some context, I imagine some contributors might be confused by "bors" as opposed to something more explicit like "Merge Queue" or "Merge Bot".

Happy to make a PR, so we don't have to source our code from a fork. :rocket:

Actually, I kind of want to change most references to "bors".

Additional context: We're using a custom COMMAND_TRIGER and editing the GitHub App to act like a chatbot. Ideally the bot does more than bors things.

So the person would say "@FoobarBot merge" and the GitHub app replies using a useful name and email for "FooBarBot".

The problem is the additional locations bors-ng uses the string literal "bors" to refer to itself whcih might confuse clients.

Maybe I'm partial to this as googling "bors" yields relevant results if you know what you're looking for. :thinking:

Anywho, still happy to PR :slight_smile: