Add tooltips for `bugs-tab` and `releases-tab` #4133

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Now that #4108 is implemented, it is time to add tooltips for bugs-tab and releases-tab, showing the bugs and releases count, just like the issue tab and PR tab. (As I mentioned in https://github.com/sindresorhus/refined-github/issues/3633#issuecomment-706185428 before.)

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Releases hit 1000: https://github.com/StylishThemes/GitHub-Dark/releases