Add rel attribute to Custom Button

[asjmcguire]

As people are starting to explore the Fediverse, they are jumping between instances - of both different Mastodon servers, but also different Fediverse projects like Calckey etc.

Could we have the ability to either have multiple Mastodon buttons, so we can be verified at multiple instances, or perhaps we could use the custom button facility, and just add the ability to have the required rel="me" attribute on custom buttons?