[FIXED JENKINS-15340] Ask for confirmation before reloading config from disk,...

[FIXED JENKINS-15340] Ask for confirmation before reloading config from disk, as it may be expensive.
First of all, /reload should require POST (also prevents XSS DoS attacks).
Adding <l:confirmationLink> to encapsulate a link that requires confirmation upon clicking.
Allowing a ManagementLink to request confirmation (also implying POST); could be used also for e.g. /quietDown.
Enhancing <l:task> to support confirmation as well, and honoring this in context menu items.