Mass edit issues and MRs from sidebar

Resources

Description

We are currently using our old dropdowns for mass-editing issues because it's hard to combine that model with our new filters. But what if mass-editing followed the same model as editing a single issue or MR? This would make our experience more consistent and it could open the door for bringing mass-editing to the Issue Board.

Design

We remove the checkboxes from the default view and add an Edit issues button next to the Add issues button.

Clicking that button reveals the checkboxes, as well as a sidebar. It also disables the search bar and sorting dropdown, since using those elements will take the user away from edit mode.

The sidebar contains:

Number of selected issues
Update issues button. It's the main button in the form. When clicked, all changes are applied and the page refreshes, taking the user out of Edit mode
Cancel button. It closes the sidebar and hides the checkboxes. It also re-enables the search bar and sorting dropdown
A section for each kind of attribute. Each section shows the name of the attribute and a dropdown. The dropdowns work the same way as in the current implementation

cc @victorwu