Add new github action to replace travis-ci (includes dependency upgrades)

[willgearty]

Travis-CI no longer offers unlimited testing for open source projects, so we need to migrate to Github Actions. Github Actions only supports Ubuntu 16+, and since @milescalabresi is moving us to Ubuntu 20 on AWS, I decided to go with that, which then required updates to node.js (v.0.10.x to v.14.x LTS), less (v.1.3.1 to v.1.7.5), and Bootstrap (v.2.0.2 to v.2.3.2). For the Bootstrap update, I merged all of our custom changes and did a quick glance at a few pages to make sure everything looked ok. The node.js and less upgrades shouldn't cause any notable/visible changes.

Everything runs smoothly on Github Actions (including all tests passing, woo!). Everything also runs smoothly on my dev server which is also on Ubuntu 20 (see notes below and new upgrade instructions in vagrant.rst). Following this, all developers will need to migrate to new vms.

Given all of these changes, it should be noted that it's very possible (and almost 100% likely) that main will never work again on the current production server (including dev3). Of course, cherry picks should still work for individual chapters, but the entire development branch will probably not work.

In terms of the LGTM and CodeQL alerts, they all have to do with Bootstrap, so I say we just ignore those.

Notes on how I created the new dev VM (I ran into numerous issues trying to update our old dev VM):

1. I started with the VM here: https://app.vagrantup.com/bento/boxes/ubuntu-20.04 (so set config.vm.box to bento/ubuntu-20.04 in your Vagrantfile)

Once you've got the VM running, you'll then need to modify the partition scheme such that there is an unmounted partition named keep_1 within the vgvagrant volume group. However, by default, the root partition in this volume group takes up the full space, so we'll need to shrink it. Unfortunately, we'll need to unmount this root partition and can only do that if we are running the server from the Ubuntu install CD.

2. Download the Ubuntu install .iso here: https://ubuntu.com/download/desktop.
3. Open VirtualBox and click on the VM that you just installed in step 1. Then click on the "Settings" button. Click "Storage", then, next to "Controller: IDE Controller", click the "Add optical drive" button. Click "Add" and browse to the Ubuntu install .iso file you just downloaded. Then click "Choose". Now click on "System" and move the "Optical" drive to the top of the boot order by clicking it and clicking the up button (and make sure the "Optical" drive has a checkmark).
4. Run the virtual machine (username should be ubuntu with no password).
5. Use the arrow keys to highlight the first option ("Try ubuntu" or something like that) and add this to the end of the text line that appears systemd.unit=multi-user.target. Then hit the enter key. (https://askubuntu.com/a/943701/1174698)
6. Follow the instructions here to shrink the root partition (you shouldn't need to do the vgchange line). I used -10G for lvreduce to shrink the partition by 10 GB.
7. Power off the virtual machine. Go back into the VM settings and restore the hard disk to the top of the boot order. You should also remove the Ubuntu .iso from the Storage options.
8. Now start the virtual machine again (username and password should be "vagrant").
9. Now run the following line to create a new unmounted partition: sudo lvcreate -l 100%FREE -n keep_1 vgvagrant (https://www.digitalocean.com/community/tutorials/how-to-use-lvm-to-manage-storage-devices-on-ubuntu-18-04#creating-a-logical-volume-from-all-remaining-free-space).
10. Optional: run update_deps.sh to update your dependencies.
11. Export the VM by running vagrant package (this is back in your own file system). You're done!

Fixes #2144 and fixes #2560.

[kkbrum]

Worked for me, and I logged in, set up a program, imported all the stuff, teacherreg, and studentreg. I think that's a good enough initial testing run. My one comment is to maybe save a copy of main as "SR 12.5" or something so that we have a workable copy if needed before merging this. Up to you though!

[willgearty]

OK, I've made a backup branch named "main-backup" in case we need to go back.