Phase status fixed

@ mention of reviewers`

@Didayolo`

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

Phases api (example: https://www.codabench.org/api/phases/1215/) was giving null status and status was not used in the UI to allow or block user from submiting to a phase which has started/ended etc.

Now this is fixed.

Issues this PR resolves

#931 (closed)

What is changed?

• Phases api ( https://www.codabench.org/api/phases/) fixed to add a valid status instead of null
• Used these 3 statuses (1. Previous, 2. Current, 3. Next)
• Serializer modified to return correct status
• Phase start/end logic based on start date and end date replaced by status in the UI submission panel
• Admin as well as participants are allowed to see submission panel
• error/warning messages are displayed to both admin and participant for not started and ended phases

Screenshots

API with valid status:

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