Handle stretch and optional times

**Which module(s) and week(s) does this change affect?**
all, mainly prep type views but philosphical

**What is the work that needs to be done?**

Figure out a way to express time trainees need to plan into their week for mandatory work as different from projected times of any activity, like stretch or optional extensions.

**Why is this work important to do?**

Time is very important at CYF. We must make sure our coursework can be completed within 20 hours during the week and workshops can be done in the planned time on class days. [How is Enough Time Defined](https://docs.google.com/document/d/1roGhtHmJccg4kJSraRcWhmGzETbeFcdfTK86iyZVZ4E/edit?usp=sharing)

Of course people work at different rates, but we track the **maximum** times someone should be taking and still actually working at the expected level.

We also offer many stretch goals, which shouldn't go towards a maximum total. But setting 0 or no time on these activities is confusing.

**Additional context**

**Who might need to know about this change?**

@illicitonion

Start with the issue's description of prep type views and read the linked “How is Enough Time Defined” document. Define how mandatory, stretch, and optional activity times should be represented and verified across the curriculum; done means the distinction is agreed and consistently applied.

Written by the indexing model from the issue text.