Improve roster creation flow

[shaunakpp]

This is a high-level issue of the three part proposed changes that for improving the Roster creation process. With LMS integrations, rosters have become an essential part of classroom and we could work on improving this experience for teachers. Here are the following improvements that we could work on:

• Move roster updates(Moves RosterEntry update to background job #2233 ) and creation to background jobs(Creating jobs for roster creation #2242)
• Move/Copy the link to roster management from settings to a tab on classroom show page. ([WIP] Move rosters to a new tab and rename to students #2274; See mockup)
• Rename rosters to students on the UI. ([WIP] Move rosters to a new tab and rename to students #2274)
• Add a setup page for creating rosters, with better messages and updates (see mockup)
• Make editing of roster entries inline instead of modal
• Teachers can accept invitations to assignments just to see a student's point of view, this already happens, but is not mentioned anywhere. Should we add communication that they can do that, or figure out any other solution that can enable teachers to get a student's PoV? (see mockup)
• For v2: Roster updates via LMSes are a great version 1, but we can iterate over it by keeping rosters in LMS and classroom in complete sync. E.g. delete roster_entry of a student no longer part of the roster in LMS.

cc: @femmebot @stephaniegiang

[femmebot]

For reference, here are early wireframes I posted on Slack with initial ideas on re-organizing the Classroom and Assignment pages.