Fix unwanted label unassignment
## What does this MR do?
- When updating the milestone
- [x] Do not remove labels when assigning a milestone
- [x] Do not remove labels when unassigning a milestone
- [x] Do not remove labels when assigning a milestone and adding another label
- When toggling selected issues labels should be kept
- [x] Select an issue with an assigned label -> pick another label from dropdown-> unselect the issue -> select the issue again -> submit the form: Existing label should not be removed.
## Are there points in the code the reviewer needs to double check?
Labels should not be added or removed to issues when doing bulk actions unless we explicitly select a label from the dropdown
## Does this MR meet the acceptance criteria?
- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
- [x] Added for this feature/bug
- [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
See merge request !4863
Fixed issue with bold in issuable sidebar
## What does this MR do?
~~The sidebar contained both normal font weight & bold font weight, so this standardises them to bold.~~
After looking at the designs, i've updated the font weights & colors throughout the sidebar to correctly match them
## Screenshots (if relevant)
See merge request !4398
Correctly shows label errors in dropdown
## What does this MR do?
Correctly shows any errors in the dropdown when creating a new label
## What are the relevant issue numbers?
Closes#18344
## Screenshots (if relevant)
See merge request !4553
It concats the duplicate labels into a single label in the dropdown with
the color being a gradient of the differnet colors being concatted.
Closes#16555
Fixes text color on labels in sidebar
Previously the labels in the sidebar would just have `#FFF` text color which could cause problems with a light background color.
With this, the text color comes from the JSON.
See merge request !3846
Set tooltips for new added labels
Fixes#15093
See merge request !3667
Added back upcoming to milestone filter
Fixed bug that would cause the currently selected value to disappear on filters
Fixed bug that throw an error when filtering by upcoming when there is only a milestone in the past
Jacob SchatzandRobert Speicher
Phil Hughes
Alfredo Sumaran
Robert Speicher
Arinde Eniola
Jacob Schatz
Jacob SchatzandJacob Schatz