Bulk edit cards

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Is your feature request related to a problem? Please describe.
I often want to apply the same field value to multiple cards in a board. Eg assign a user to these given 5 cards, or set the due date on these 3 cards. Presently I have to go through the edit process for each card individually.

Describe the solution you'd like

• Hold down ctrl (or long press on mobile) to enter multi-select mode.
• As you click on each card it becomes highlighted along with the others in the selection.
• In multi-select mode a new button appears at the top "edit selected...".
• Clicking this button opens a limited version of the add/edit card interface, with only the fields which it makes sense to bulk edit
  • Separate fields for add and remove tags
  • Separate fields for add and remove users
  • Due date
  • Add attachment
  • Add a comment.

Describe alternatives you've considered

• Azure DevOps and Jira have another UI approach to the bulk edit interface, where the user selects fields to modify from a dropdown. e.g.
  
  Note that "comment" is included by default, since good practice in a project management environment would be to leave a note if you make bulk changes. That is probably not the case for Deck.

• It might also be interesting to allow default values on a per-board level. e.g. if the entire project should be assigned to one user, or have a given due date. I wouldn't consider it a part of this feature request, however.

Review the existing board card selection and card add/edit interface first; the issue does not name specific files or tests. The feature is done when users can select multiple cards, open a limited bulk-edit interface, and apply the listed tag, user, due-date, attachment, and comment changes, including mobile selection.