Fix off-by-one applicant free time & days available figures

[thibaudcolas]

Based on the "Time commitment in application" discussion thread in chat, where Sage reported the off-by-one numbers are a known issue. This can be reproduce with the ready-made InternshipWeekScenario, with week 1.

For coordinators / mentors this appears in two places:

• Looking at individual applicant info, "Applicant has 101% of their time free"
• Looking at the table with all applicants, under "Number of days available", "89 / 88 days"

Screenshots for reference:

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The calculation of the internship length seems to be inclusive of both the start and end date, without needing the + 1.

I chose to remove the increment because there are already two other places in the code where the same calculation is made without it:

website/home/views.py

Line 1994 in d2ced82

internship_total_days = current_round.internends - current_round.internstarts

website/home/models.py

Line 2840 in d2ced82

internship_total_days = current_round.internends - current_round.internstarts

That second occurrence in particular is what’s used for both calculations that were showing the off-by-one error.