Replace # and % in Image URLs to fix loading of images with these characters in the fileName

Cannot use encodeURI/encodeURIComponent because they replace `#` with `%25`, which is incorrect - should be replaced by `%23`.

I'm not sure if there is a Brackets' utility for this already (I searched for it but couldn't find it).

I've tested with `test#tt.jpg` and `test%tt.jpg`, but nothing else - are there any other special characters that should be added to this replace scheme?

Should this be moved to a module in `utils/` and loaded from ImageViewer?

Fixes #9117.
Should we be replacing `/` with `%2F`? Since this is not a valid symbol in a file name (directory separator), perhaps this should not be replaced?