PHP 8 support (closes #23)

[gregstoll]

This updates the code to support PHP 8. Note that it's worked for everything I've tried on my local install, but it's definitely possible there are more problems lurking - feel free to open issues as you find them!

Categories of stuff fixed in this PR:

• Instance methods can no longer be called with a static syntax (i.e. ClassName::instanceMethod()) - this is mentioned here but I can't find the original deprecation notice. This has been deprecated for a while but now it returns an error. Made a bunch of methods static to fix this. In some cases a method was called with an instance syntax some places and a static syntax others, so I had to make two separate functions. (for example commit 7e768be)
• Curly braces can no longer be used to index into arrays and strings - see deprecation notice. Fix was simple but this was done in a ton of places. (for example commit 5a584b5
• Constructors used to be able to have the same name as the class but now have to be named __construct() - see deprecation notice. Another simple fix needed in a ton of places. This one was scarier because constructing an object would still work, but the constructor wouldn't be called, which would lead to unpredictable problems later on. I search and tried to make sure to find all of these. (for example commit c30f191)
• Method prototypes of methods on a subclass have to match the prototype of the parent class's method. (can't find a reference to this) This was mostly a matter of adding an extra parameter (for example commit f00650e, although there was one case where two unrelated methods had the same name and I changed one (GalleryDynamicAlbum::createDynamicAlbum() in commit 5aac894.
• create_function() no longer works - see deprecation notice. Fix was to use anonymous functions instead. (for example commit 368b772)
• each() no longer works - see deprecation notice. Fix was to use foreach construct instead. (for example commit 6629937)
• Ternary operator (i.e. a ? b : c) used to be left-associative and you could nest them without parentheses; now parentheses are required - see deprecation notice. Fix was to add parentheses.

I also fixed a few warnings that were cluttering up the debug output.