Remove legacy XUL staff client code

[Jeff Godin]

The legacy XUL staff client was disabled as of Evergreen 3.2 in bug 1790923.

Building on that, this bug covers the removal of XUL staff client code and supporting bits (staff client installer, upgrade scripts, etc).

If needed, there may be child bugs created that reference this bug.

[Jeff Godin (jgodin)]

While it may have begun with more of a focus on the Dojo Toolkit, this document from Jason Stephenson contains information that could be useful regarding the removal of Dojo and XUL from Evergreen: https://docs.google.com/document/d/1e1I-WG96-iIeUzQUgXUtJx0dfq656emCmF_S84DxRrg/edit?usp=sharing

[Galen Charlton (gmc)]

I've started a collab branch: collab/gmcharlt/remove-xul-client

The approach I have in mind thus far is:

- remove the ability to build the XUL client
- remove places in the legacy Dojo and AngularJS code that try to invoke XUL interfaces when running inside a XUL client. Once this is complete, there should be no remaining references to isXUL() and window.IAMBROWSER
- remove the XUL chrome (and deal with any remaining references to Open-ILS/xul/staff_client/chrome/content/util/date.js, which gets a (broken) symlink from /openils/var/web/opac/common/js.date.js) There is other JavaScript that copies from chrome to server during installation that may need to be tracked down as well.
- trim Open-ILS/xul/server of everything that isn't still in use (such as the user permissions editor. It might not be a bad idea to move those remaining bits elsewere (say, Open-ILS/web/xul) so that the final results drops Open-ILS/xul from the source tree entirely.

[Galen Charlton (gmc)]

Noting several things that should go away entirely with the successful removal of the XUL client:

- xulG.new_tab
- (maybe) all references to xulG
- isXUL()
- window.IAMBROWSER
- ctx.is_browser_staff in OPAC templates (is_staff will suffice)

[Galen Charlton (gmc)]

Noting that one of the patches for bug 2051370 is a slightly revised version of the make_release patch in the collab branch.