use find_by in active_profile

[elijah-wright]

🛠 Summary of changes

this PR changes active_profile in the user model to use find_by, which should be faster. I tested my changes on my machine with a test user with four profiles and the difference was 0.449454 seconds against 0.235338 seconds. it also moves the @active_profile check to where profiles are deactivated by reloading the user model rather than calling .active, which might have been useless (if the code checks a user with a false active_profile and assigned @active_profile based on the in-memory query, @active_profile would become nil and defined?(@active_profile) would be true, but .active would be nil, bypassing the check)