Automatically change the semester

[qaisjp]

Summary: write a scheduled action that updates the semester field in the Jekyll config.

Motivation: it's March, and nobody thought to update this setting to point to Semester 2 — https://github.com/compsoc-edinburgh/betterinformatics/commit/9dfbe745a0ad3690fac254c634e82c4e94eb8a3d

We should run the scheduled action on the 29th of:

• August, to set the semester to semester 1 (this is after resits)
• December, to set the semester to semester 2 (this is after S1 exams)

Cron: 18 5 29 12,08 *

Scheduled action documentation

See also: https://github.com/compsoc-edinburgh/betterinformatics/pull/117#pullrequestreview-625213580