Finished the exofab revamp:
* designs are now using the construction time var and the hacky /obj/item/mechavars is gone
* designs updated : there shouldn't be "no material" designs anymore
* cleaned the objects of their construction_cost and construction_time vars
* fixes the material name being displayed with $ character before
* this also fixes the overlay animation in inserting sheets
* the category var is now a list, so parts can be build/used with several chassis (fixes the Firefighter Chassis appearing when building Ripley)
* properly fix the href vulnerability when emptying (you can now remove all sheets of a materiel at once)
* the exofab now won't create a material sheet if there's not enough material (e.g 1000 metal)
* the exofab now won't accept a sheet if it would overstuff it (losing some material in the process)
* cleaned the code a bit (simplified, streamlined, using global vars, etc)

Finish #5722 and fixes #5954.
Materials (resp. programming) tech now reduces the building cost (resp. building time) for the exofab
Merge pull request #6189 from Menshin/exofab_revamp_end+fixes

Exofab revamp finishing and fixes