Added a button into the sidebar for issues & merge requests to allow users to manually create todo items

Closes #15045

* make sidebar button white
* make header button gray if not primary
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

The buttons were rarely used and added at least 1 query each on every page load.

This will be useful when you want to ask for the 
number of items and later iterate over them, without 
needing to ask if the association is load or not.

So you avoid extra database queries

In the sidebar when there is a due date, a link to remove due date becomes visible

Closes #17392

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

Fixed issue with dropdown not being toggleable again

Fix typos on sorting dropdown related to due date
Remove constant array and add Structs on Issue to keep due date data to fill options

Add due_date text field to sidebar issue#show
Add ability sorting issues by due date ASC and DESC
Add ability to filtering issues by No Due Date, Any Due Date, Due to tomorrow, Due in this week options
Add handling issue due_date field for MergeRequest
Update CHANGELOG
Fix ambigous match for issues#show sidebar
Fix SCREAMING_SNAKE_CASE offenses for due date contants
Add specs for due date sorting and filtering on issues

Closes #15186

# Conflicts:
#	app/assets/javascripts/labels_select.js.coffee

These were being blocked by adblocks

Closes #15043

Fix underscore template error.
Fix `=` spacing

Milestones needed a none selector
Assignee needed unassign.

Added nice animations too.