Exclude dropped students from watchlist during refresh and updates

[victorhuangwq]

Description

• Removing dropped students when refreshing the watchlist
• Exclude dropped students from being considered when conditions are updated

Motivation and Context

Addresses the open issue here: #1611

How Has This Been Tested?

Testing dropping pending student

Initial State:

I proceeded then to:

• Drop User0 from the course
• Click the Refresh button on the watchlist

I then updated the conditions (change of metric conditions in the student metrics tab):

I proceeded then to:

• Undrop User0
• Click the Refresh button on the watchlist

Testing dropping contacted / resolved student

I resolved User0 (using the resolve button):

I then proceeded then to:

• Drop User0
• Click the Refresh button on the watchlist

I then updated the conditions (change of metric conditions in the student metrics tab):

I then proceeded then to:

• Undrop User0
• Click the Refresh button on the watchlist

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting
• My change requires a change to the documentation, which is located at Autolab Docs
• I have updated the documentation accordingly, included in this PR

Other issues / help required

If unsure, feel free to submit first and we'll help you along.

[20wildmanj]

Followed testing strategy, functionality works as specified. LGTM