[Merge] lp:~ahayzen/webbrowser-app/migrate-to-tabs-component into lp:webbrowser-app/staging

[Andrew Hayzen]

Fixed the issues with the snapcraft, at the moment ui-extras isn't within u-a-p (maybe we can request it if we think it is worth it?)

Removed the top margin, and set the height of the tabs bar to be 3GU - which is the same as the ubuntu-terminal-app and what the design spec says.

Please rereview :-)
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https://code.launchpad.net/~ahayzen/webbrowser-app/migrate-to-tabs-component/+merge/312340
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