Resolve "Claimed slots page for unit shows also past claimed slots"

What does this MR do?

Makes sure the unit slots being shown are only for today or the future. The reason why I don't hide ones that have already passed but where still due today is that there may have been operational delays in the hospital. We still want nurses to be able to use those slots, even if they've passed.

Closes #264 (closed)

Does this MR meet the acceptance criteria?

• I have added tests to validate the fix.
• I have updated the documentation accordingly.
• I have added translations for all new client-side text.
• I have updated the changelog accordingly.