Create a tab group

[mheubusch]

As a user, I want to save the sites in my current browsing session as a group so that I can organize information in a way that is meaningful to me.

Acceptance criteria

• I can select some or all of the open tabs in my current session
• I know how many tabs I have selected
• I can save the selected tabs as a discrete group
• I can access the group and see which sites were saved to the group without opening all of the sites into a new browsing session

┆Issue is synchronized with this Jira Task

[sv-ohorvath]

@sblatz In the mockups (https://mozilla.invisionapp.com/share/8ZQVRNUJW3H#/screens/359019534) I see that a new collection created should show a purple border animation.
And there is one issue that I also noticed, #2412 (the save collection button is missing).
Were this changed in the meantime?

[sblatz]

@sv-ohorvath the "save collection button" should be available in tomorrow's build 😄

The purple animation should be covered by #2359

[sv-ohorvath]

Thank you @sblatz! I can close this now :)

[sv-ohorvath]

Reopened to add new blocking issues.

[sblatz]

The blocking bugs should be fixed by #2748.

[sv-ohorvath]

All blocking issues were fixed.