Context: https://meta.discourse.org/t/stop-appending-username-to-url-when-badge-system-disabled/164904
Share links for topics and posts have the current username appended to them when the user is logged in.
If badges are disabled, those added usernames are not beneficial since they're only used to track progress towards certain badges.
This PR makes two change.
1. it moves the logic for the share link url to a centralized helper because it's shared in both topic and post models.
2. it stops username params from being added to share links when badges are disabled.
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