Frontend improvements

Some better Button titles & arrangements & default settings

Use cases

• Tutors & reviewers could be better guided through the whole process
• reviewers start less biased in the validation process.

Links / references

Feature checklist

• Login: exchange order of Register & Login buttons (I observed myself several times to hit the Register button first, although I am registered)
• Session: rename Students into Participants (more general and more applicable)
• Session: rename Feedback-Button into Feedback history-Button
• Session: initial setting for validation should be all feedbacks hidden

(Make sure these are completed before closing the issue, with a link to the relevant commit.)