Add link to recommended servers to Server check notifcation

[davidhedlund]

Is your feature request related to a problem? Please describe.

Server check
This server has been unreliable in the past or had other issues. We recommend choosing another server. You may not be able to subscribe to this server, or you may not receive notifications.

Describe the solution you'd like
Add https://github.com/nucleus-ffm/foss_warn/wiki/What-is-UnifiedPush-and-how-to-select-a-distributor#for-nfty-which-default-server-should-i-use to the notification message in the app like this:

Server check
This server has been unreliable in the past or had other issues. We recommend choosing another server. You may not be able to subscribe to this server, or you may not receive notifications.

[davidhedlund]

• Some recommended ntfy servers cannot be used with FossWarn #274