PMI choice selection message should include an option to set the choice as the user's default

[mickmister]

At the moment, the user needs to use a slash command to change their default setting of using PMI or unique meeting id. We can instead provide a checkbox in the post to determine whether the user wants to make the which option their default preference.

https://www.loom.com/share/aafd314ee9744fc0b3c47b1c17b67662