RoadmapEpics align with start of month in "By month" view

Summary

On the Roadmap view the epics will align with the start of the month in stead of the specified date when viewing with the "By month" view options

Steps to reproduce

1. Create epics with start dates staggered throughout January
2. View Epics Roadmap with date range = "This year", and "By month"
3. Epics align to start of month in stead of the set start and end date.

Example Project

What is the current bug behavior?

Epics are aligned to the start of month in stead of the described start date

What is the expected correct behavior?

Epics should be aligned to their set start date

Relevant logs and/or screenshots

Incorrect alignment of epics

Correct alignment of epics, only correct on setting: Date range = This quarter

Output of checks

This bug happens on GitLab.com /label reproduced on GitLab.com

Results of GitLab environment info

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(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

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(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes