Add closing notice to homepage 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 three items:

• a visible banner about the migration with text template covering all important information
• a visible link to the replacement vspd server. If not provided, link to decred.org/vsp is shown to users.
• a link to Decred Blog about the new vsp implementation.

dcrstakepool operator can set the link to their vspd and tweak banner text (specify a closing message). However, the closing notice is only shown if the dcrstakepool operator is closed.
Pictures below:
Before:

After(If link to new vsp server is set):

After(If link to new vsp is not set):

After(If custom message is not specified and new vsp not set):

Closes #635

[jholdstock]

Hi @ukane-philemon,

This is good work, but unfortunately we don't plan to release any new versions of dcrstakepool so this change is unlikely to ever be deployed by operators. I will revierw and we can get it merged regardless, just in case somebody does find it useful at some point.