phase start time and end time added

[ihsaan-ullah]

@ mention of reviewers

@Didayolo

A brief description of the purpose of the changes contained in this PR.

Now users can select custom time for phase start and end in UTC+0 time. This can be done from both the yaml file and the editor

Screenshots

You can mention the start and end time in your yaml like this (in 24hrs format and UTC+0 timezone)

When you upload the bundle with this yaml file, you will see the phase dates in your time zone (my zone UTC+5)

Adding new phase in the editor (empty form)

Adding new phase in the editor (filled form)

Edit phase form

Documentation

Documentation updated here: https://github.com/codalab/codabench/wiki/Yaml-Structure#phases

Issues this PR resolves

• Changing the start/end time of a Challenge #1155 -> Phase start/end time:
• Phase end time is not changeable #1598

A checklist for hand testing

• Upload a version-2 bundle without time (only dates) for phases and check that phase date-time is correct (default time used 00:00)
• Upload a version-2 bundle with time for phases and check that phase date-time is correct
• Use competition UI to add a phase with different date-times and check that everything makes sense

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge