Add deprecation notice and vspd link to UI home page

[xaur]

Similar to decred/vspd#224, I suggest to incentivize and facilitate the migration by informing dcrstakepool users about it on the front page.

This includes at least two items:

• add a visible banner about the migration with text template covering all important information (sunsetting schedule, link to new decrediton, link to dcrdocs page, etc)
• add a visible link to the replacement vspd instance

dcrstakepool operator should be able to set the link to their vspd and tweak banner text (e.g. to post specific sunsetting schedule), but in general we should try to prepare common stuff for them to minimize their work and minimize user confusion during the process.