Specify wich table to select data from in join query

[iPurpl3x]

Changes proposed in this pull request:
Added a select(...) instruction to queries that join users and tag_user.

This shouldn't change anything for the extension as it is. But in my case, I have added columns to tag_user (using another extension) that would override columns of the users table when the join happens because the columns have the same name in both tables. Explicitly telling Laravel what table to select data from (users) fixes that issue.

Reviewers should focus on:
Nothing is broken. Notifications still go out normally to tag followers.

Confirmed

• Backend changes: tests are green (run composer test).

Required changes:

[clarkwinkelmann]

Tested locally, no issue