Create TF-1640-User-can-assign-label-to-an-email.md (#1668)

(cherry picked from commit d349a3140bf4e7d40d468349b5440e439e8bf733)

docs/stories/EPIC-Label/TF-1640-User-can-assign-label-to-an-email.md

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+# Summary
+
+* [Related EPIC](#related-epic)
+* [Definition](#definition)
+* [Screenshots](#screenshots)
+* [Misc](#misc)
+
+## Related EPIC
+
+https://github.com/linagora/tmail-flutter/issues/1638
+
+## Definition
+
+**UC1. Assign a label for messages in thread-view**
+
+- Given that I am a Tmail user and I have logged in successfully
+- On thread-view, I select one or multiple messages
+- On action bar, I can see an icon Label
+- I click on this icon, there will be a popup that listing all current labels
+- I can select one or multiple label then click button Apply
+- Then selected labels will be appeared on message as tags, on both thread-view and message content view
+
+![image](https://user-images.githubusercontent.com/68209176/228160592-91c5428e-0e36-4edc-a589-ea474408c0e1.png)
+
+**UC2. Assign a label for messages in message detail view**
+
+- Given that I am a Tmail user and I have logged in successfully
+- I click on one message and the content will be opened
+- On action list on top of screen, I can see an icon Label
+- I click on this icon, there will be a popup that listing all current labels
+- I can select one or multiple label then click button Apply
+- Then selected labels will be appeared on this message as tags, on both thread-view and message content view
+
+![image](https://user-images.githubusercontent.com/68209176/228160873-4046f365-8693-45a1-b088-38b248ddb964.png)
+
+
+**UC3. Remove a label on a message**
+
+- Given that I am a Tmail user and I have logged in successfully
+- On thread-view I click on one message that currently have some label, the content view of this message is opened.
+- On content view, I can see labels of this emaill, each label contains a cross icon
+- I click on Cross icon, 
+- The label is removed from this message and there will be a toast notification 
+“[Label name] is removed from this email]” and Undo button
+- If I click Undo, the label is back to this email again.
+
+![image](https://user-images.githubusercontent.com/68209176/228167740-50be89b5-f3d4-44b3-8f22-012848ade474.png)
+
+![image](https://user-images.githubusercontent.com/68209176/228920266-5f398a0b-45bf-44ef-904a-aa3016c53bc3.png)
+
+
+[Back to Summary](#summary)
+
+## Screenshots
+
+None
+
+[Back to Summary](#summary)
+
+## Misc
+
+None
+
+[Back to Summary](#summary)