Add ability to set host instance

[Manishearth]

Following accounts on a separate instance from yours is a multi step process and can be slow.

However, it's easy when you open that account on your own instance, e.g. if logged in to mastodon.social, https://mastodon.social/@[email protected] is easier than opening hachyderm.io/@ekuber and following from there.

There is a bulk import feature, however when using Debirdify I found myself wanting to check individual accounts to see how active they were. Ideally, those links are on my own instance, so i can quickly follow if i need. Could be a simple text box i set at the top to change all the links

Thanks for making this, it was very useful! And I appreciate making it easy to log out and revoke permissions.