bf-imagej.sh: generalize most of the script

This change renames variables and reorders a few things, to centralize
Bio-Formats-specific configuration into a single "Parameters" section at
the top of the script. The idea is that in the future, this script can
be split into a more general-purpose update site deployment tool, which
can be called by e.g. "Bio-Formats 4 daily builds" and "Bio-Formats 5
daily builds" deployment jobs without violating the DRY principle.
Split Bio-Formats uploader into BF4 and BF5

This allows us to use nearly identical logic for uploading builds
for both Bio-Formats dev_4_4 (4.x) and Bio-Formats develop (5.x).
Fix inter-script references

We need to use dirname so that the upload-bf*.sh scripts can call
upload-update-site.sh even when executed outside the bin directory.