Add dropdown with popular Mastodon instances

[aitorres]

When the user wants to log in, they need to manually type in the domain of their Mastodon instance / server, e.g. mastodon.social. It'd be beneficial to have a dropdown that lets the user quickly select one of the most popular instances + the ability to type-in their own instance if it's not found on the list.

There's no need to call a third-party API or service in order to get popular instances, they can be hardcoded for now. Some possible choices:

• mastodon.social
• mastodon.online
• mstdn.jp
• mastodon.cloud
• fosstodon.org

If possible, an icon could be added to each entry (maybe their favicon).

Remember to still allow any user to type in an instance not found in the list!