LICENSE.md: Bump the CC BY 3.0 to 4.0

Titles from the v4 release notes [1]:

* A more global license
* Rights outside the scope of copyright
* Common-sense attribution
* Enabling more anonymity, when desired
* 30-day window to correct license violations
* Increased readability
* Clarity about adaptations

[1]: https://creativecommons.org/version4

Using template variables everywhere

- Fixed error in header.html; missing }

* 'master' of https://github.com/tbekolay/swc-shared:
  Initial version of shared files

Merging to get `tools/update` so that this pull is automated

This is up for discussion, but at the moment is the
minimal set of files needed to be a Software Carpentry lesson.