Add Model to handle Burndown Chart

[Jangsoodlor]

• Add a new model, EstimateHistory that can keep track of changes in a taskboard during various days
• Add tests for that new model
• Add initial burndown chart page. Currently, it renders Lorem Ipsum text but will be work on later.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
manager/views.py	66.66%	1 Missing :warning:

Files with missing lines	Coverage Δ
manager/models.py	96.42% <100.00%> (+5.51%)	:arrow_up:
manager/urls.py	100.00% <ø> (ø)
manager/views.py	95.58% <66.66%> (-0.69%)	:arrow_down: