some minor refactorings

- Use rails new `find_by` instead of `find_by_commit` 
Tip.find_by(commit: commit.sha).nil? instead of Tip.find_by_commit(commit.sha).nil?
- Use new Ruby Syntax.. :smile:
- Use Arel not instead of hardcoded string queries. `where.not(refunded_at: nil)` instead of `where('refunded_at is not null')`
- In user model the generate_login_token is renamed from generate_login_token! because it is not generating any exception as such. It is a common practice to name the methods raising exception with a bang like save!, create!, etc.