Feature: Time LImits (testing the curriculum)

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

### prep

Given a prep view
And a time_limit_prep set in hugo.toml or an override time_limit_prep set in the page front matter

When the total time of blocks set on prep exceeds the time limit
Then the build should fail
And the error should explain too much work has been assigned

GIven a prep view
And no time_limit_prep assigned
Then the build should not fail

### day plan

Given a day plan view
And a time_limit_day set in hugo.toml or an override time_limit_day set in the page front matter

When the total time of blocks set on the day plan exceeds the time limit
Then the build should fail
And the error should explain too much work has been scheduled and the day is too long

GIven a day plan view
And no time_limit_prep assigned
Then the build should not fail

### backlog

Dependent on time being extracted from issues, which does not yet exist, so possibly this could be done separately

Given a backlog view
And a time_limit_backlog set in hugo.toml or an override time_limit_backlog set in the page front matter

When the total time of issues showing on backlog exceeds the time limit
Then the build should fail
And the error should explain too much work has been assigned

GIven a backlog view
And no time_limit_backlog assigned
Then the build should not fail

Note that some backlog items are labelled as stretch, and don't have time allocated to them. If the time allocated section contains the (case-insensitive) string "stretch", we should skip it.

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

https://common.codeyourfuture.io/common-theme/front-matter/#time

Time is very important to our learners, as many people are fitting learning around other commitments, including multiple jobs, childcare, and other responsibilities. If we are asking someone to do an activity, we must be clear about how long it will take, so they can find that time.

**Additional context**

#738

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