Add -buildroot patch and only build cython interfaces once.

Also:
- Drop LANGUAGES variable setting, now ignored by the sagemath build system.
- Drop unused SAGE_CBLAS variable from /usr/bin/sage.
- Do not force the C locale when launching sagemath.
- Allow the user to override SAGE_DEBUG in /usr/bin/sage.
- Add -ecm, -giac, and -latte patches to fix interactions with external tools.
- Add -sigfpe patch from upstream.
- Fix flask_autoindex and flask_openid imports in sagenb.
- Add Education category to the desktop file (bz 1624545).
- Fix SINGULAR_SO variable in /usr/bin/sage (bz 1636759 and 1655248).
- Invoke twistd-2 instead of twistd (bz 1640890).
- Improve jupyter integration (bz 1663165).
- Move existing jupyter integration into the notebook subpackage.
- Require python-jupyter-filesystem instead of owning its directories.
- Drop one more remnant of the F24 to F25 upgrade fixup.

sagemath-latte.patch

@@ -0,0 +1,21 @@
+diff -up src/sage/interfaces/latte.py.orig src/sage/interfaces/latte.py
+--- src/sage/interfaces/latte.py.orig	2018-08-03 05:10:08.000000000 -0600
++++ src/sage/interfaces/latte.py	2019-01-18 11:32:37.859220860 -0700
+@@ -112,7 +112,7 @@ def count(arg, ehrhart_polynomial=False,
+     # Check that LattE is present
+     Latte().require()
+ 
+-    args = ['count']
++    args = ['latte-count']
+     if ehrhart_polynomial and multivariate_generating_function:
+         raise ValueError
+     if ehrhart_polynomial:
+@@ -304,7 +304,7 @@ def integrate(arg, polynomial=None, algo
+     # Check that LattE is present
+     Latte().require()
+ 
+-    args = ['integrate']
++    args = ['latte-integrate']
+ 
+     got_polynomial = True if polynomial is not None else False
+