[FIXED JENKINS-17330] FilePath.installIfNecessaryFrom should avoid routing download over remoting channel.

Otherwise you get nonsense like this:
Cannot delete this build, because it is #3 depends on this

confirmDelete (and confirmDeleteAll) already say why a build cannot be deleted (and prevent you from proceeding), so no need to hide the Delete link in this case.

Conflicts:
	changelog.html

Revert "[JENKINS-17337] Added test for NPE.(cherry picked from commit 1bf958af)"

This reverts commit 18b5f4f1.
It looks like this change depends on other test harness related changes.
It doesn't build on this branch.

[JENKINS-17337] Added test for NPE.(cherry picked from commit 1bf958af)

[FIXED JENKINS-3265] Hotfix for regression in matrix project loading from 25084f52.(cherry picked from commit 3370e6ed)

[FIXED JENKINS-7291] Permit flyweight tasks to run on master even when it has zero configured executors.

Always adding Computer for master as a fallback

The original proposed fix for JENKINS-7291 creates a Computer object
transitively. This seems unwise as it violates the design of Computer
as stated in the javadoc, and for example we can end up creating two
Computers for the master.

I think a better fix is to create a Computer for the master all the
time, even if there's no executors configured. The discrimination in
Queue.makeBuildable would ensure that such phantom Computer is only used
as a last resort (statistically speaking).

I've also tweaked executors.jelly a bit. I simplified it somewhat
based on the idea that "if there's only one computer to show, the
context is likely making it obvious".

(I must be missing the intricacy in the current code.)

Originally developed in a branch at
2c5b57fcc1f39ed39057254e802f4183db5aa0dc then squashed for clarity.

as I was playing with it to understand what those logics mean.

aa432839.　

d8b29df3.　

f6a2f64b.　

5537213f.