PL: teacher application now requires numbers for length of class

[breville]

Just over 14% of the 19-20 teacher applications have a non-numeric entry for cs_how_many_minutes, cs_how_many_days_per_week, or cs_how_many_weeks_per_year - the three entries that are multiplied together (and divided by 60) to get cs_total_coursehours. This leads to 0 total course hours, and then the field's not visible in the application details.

Firstly, this change forces the field to be shown in the application details, regardless of its value. Here's a sample:

Secondly, this change enforces numbers in those three fields in the teacher application at the time of entry by the user, and gets the benefit of a recent improvement to that input enforcement. Numbers with an optional decimal point are accepted.

The existing logic to truncate the values to integers and multiply them together is unchanged. Here's a sample result in application details (9 * 9 * 9 / 60 = 12.15):

There is one known issue with this approach. For a user resuming an existing teacher application which already has non-numeric content in the relevant input field, attempting to modify one character at a time (e.g. by backspacing) will have no effect if the resulting text is still non-numeric. The workaround is to select the entire field and delete it at once. This feels rare enough to not be worth special-casing, but wanted to call it out.