Add closing notice to home page UI

[ukane-philemon]

This PR incentivize and facilitate the migration from dcrstakepool to vspd by informing dcrstakepool users about it on the front page.
This includes at two items:

• a visible banner about the migration with text template covering all important information
• a visible link to the replacement vspd instance. If not provided, link to decred.org/vsp is shown to users.
  dcrstakepool operator can set the link to their vspd and tweak banner text (specify a closing message)

Pictures below:
Before:

After(if new vspd link is provided)

After(if closing message not specified and if new vsp link is not provided)


Closes decred#635