[Respects] Refactor payRespects() using humanize_list()

[quachtridat]

Cog

Respects.

Refactor

cogs/respects/Respects.py:300-324 forms a respect message by counting the number of members who have paid respects, and formats the resulting message for cases when there is only one member, when there are two members, and when there are three people or more.

However, the manual work done by these lines can also be done in a cleaner way by making calls to redbot.core.utils.chat_formatting.humanize_list(). Also, this may as well help make the code look cleaner to work with #482 because it's likely that issue will also involve modifications towards the same section of the code.