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

Increased z-index of right sidebar

Fixes issue with active button overlapping it

Closes #15243

See merge request !3722

Add margin to top of form actions

Closes #15213

Also fixes other modal inputs (like add new directory, etc)  

![Screen_Shot_2016-04-20_at_1.47.14_PM](/uploads/14b474da2f86dcc3c1afd9e8ea09e812/Screen_Shot_2016-04-20_at_1.47.14_PM.png)

See merge request !3841

Mutliple label filter

Fixes #989 

See merge request !3438

Removed duplicate ID's from the HTML



See merge request !3818

Update changelog



See merge request !3801

fix bug causing the infinite scrolling in activity feeds to break when you click a link and navigate back

![arrg_](/uploads/53b2a01d3b55938cc12c805e1a81ff6b/arrg_.gif)

See merge request !3835

Discussion note icon shows up when you click on diff on touch screen

Fixes #15104 

* Fix bug that stopped discussion note button from showing up on iOS
* Shifted button a few `px` to fix slight cutoff
<div>
<img src="/uploads/9d507091f33aa69ef68323386bf3a5c2/Screen_Shot_2016-04-15_at_12.08.46_PM.png" width="300px">
<img src="/uploads/a964a2a46636cbe4593abf272b5796f7/Screen_Shot_2016-04-15_at_12.11.40_PM.png" width="300px">
</div>

See merge request !3755

fix bug causing the infinite scrolling in activity feeds to break when you click a link and navigate back

Backport select_helper from EE



See merge request !3825

Fixed issue with no label not working correctly

Fixed issue with sidebar edit links toggling anchor

This caused an issue with the page scrolling from the bottom to the top. Also stopped the dropdown links doing the same.

See merge request !3819

Render relative timestamps for all commits in projects commit list

fixes #15366 

See merge request !3821

Created issuable singleton to house the filtering

Removed commented text from system notes

![Screen_Shot_2016-04-20_at_14.49.22](/uploads/b1617af49cc35595afd6dfcf761a775c/Screen_Shot_2016-04-20_at_14.49.22.png)

See merge request !3836

Fixed issue with new MR form layout

Currently it is all over the place:

![Screen_Shot_2016-04-20_at_14.59.48](/uploads/09c3dffc2138140e3b009ab2564f0aca/Screen_Shot_2016-04-20_at_14.59.48.png)

Now:

![Screen_Shot_2016-04-20_at_14.59.44](/uploads/ef5ba8b7393253ecb732c57ba37dd0cf/Screen_Shot_2016-04-20_at_14.59.44.png)

See merge request !3837

Fixes issue with note form inheriting text color

It was only really noticeable on dark themed diffs

![Screen_Shot_2016-04-20_at_14.44.58](/uploads/4f32b487de7e4c09a494dc3c8c767ece/Screen_Shot_2016-04-20_at_14.44.58.png)

See merge request !3834

Verify the MR being displayed question has a merge commit set and can be cherry-picked

See merge request !3830