First reorganization

[embray]

This is a first set of changes to reorganize the sage images, including the makefile, their dockerfiles, and the sage installation script that runs in the containers. This doesn't do everything I want yet, but it partially addresses:

• Optimize the size of the images #3 -- many unnecessary files have been removed, reducing the sizes of the sagemath and sagemath-develop images from ~7 GB to ~4 GB. More could probably be done but that's a good start.
• Install the startup scripts in ~sage/bin #6 -- all files used by sage are installed with full permissions granted to the sage user. By default sage is installed to /opt/sage/ (the sage user does not have permissions on /opt/ but they can read/write to /opt/sage/. Their DOT_SAGE is still in the sage user's home directory). However, it is also possible to change the installation location for sage. Regardless the sage user has full permissions on the install location.
• Other minor improvements that should make future work easier.

Still to do (in a separate PR):

• The Dockerfiles for sagemath and sagemath-develop are identical except for the default value of the SAGE_BRANCH argument. I intend to eliminate the separate sagemath-develop Dockerfile, and instead add additional make targets for "develop" versions of all relevant images (currently just "sagemath" and "sagemath-jupyter"--sagemath-patchbot is of course a separate case). Building the "-develop" versions is mostly just a matter of running the build target with an additional option to change the SAGE_BRANCH variable. This would finally eliminate any duplication.

[embray]

Note--I've reworked this script so that it is run as root (before the USER sage command in the Dockerfile). This gives it freedom to perform operations that should be done as root, like creating links in /usr/bin/. However it runs the sage build as the sage user.

This means there's actually no reason for the sage user to be in the sudoers list in the first place, and we could remove that entirely (though this PR doesn't do that yet).

[embray]

The Dockerfiles for sagemath and sagemath-develop are identical except for the default value of the SAGE_BRANCH argument. I intend to eliminate the separate sagemath-develop Dockerfile, and instead add additional make targets for "develop" versions of all relevant images (currently just "sagemath" and "sagemath-jupyter"--sagemath-patchbot is of course a separate case). Building the "-develop" versions is mostly just a matter of running the build target with an additional option to change the SAGE_BRANCH variable. This would finally eliminate any duplication.

Although, that plan has a bit of a bug in it, which is that images like sagemath-jupyter wouldn't respond to a change in the SAGE_BRANCH argument, since it is built from the existing sagemath image. A hypthetical sagemath-jupyter-devel image would have to be build from a different base image. As I already found, the base image in a Dockerfile can't be changed via a build argument. So this would have to be handled differently.

For now I'll just make a special case for sagemath and sagemath-devel and worry about adding additional "-devel" images, if we want them, later.