Use relative URL's instead of absolute

[knight-of-ni]

I do not know of any negative side effects with using relative url's instead of absolute.
Let's get this into master and test.

[SteveGilvarry]

No issues with my testing of this either, happy to merge? If so I will do it.
Only question I had is with only 3 places that use ZM_BASE_URL is this part one to ensure it has not effect before removing it altogether?

[knight-of-ni]

Yeah, pretty much. I'm extremely hesitant to change things that have been working a particular way since the beginning of zoneminder. This is one of those cases. We have a really good reason to change it, but from past experience there have been those who use zoneminder in ways we don't anticipate whom we have affected. I can't think of what that would be in this case, but I still chose to play it safe.

If for some reason this merge causes an unforeseen problem, I made it real obvious for anyone to see how to change it back. We can remove the comments in a future revision.

Thank you for merging.